This module is a resource for lecturers

Defining terrorism

Before examining the meaning and associated concepts and principles of (international) criminal justice and international criminal law as they apply to terrorist crimes, it is first necessary to briefly examine some of the principal reasons for, and implications of, the absence of a universally agreed definition of terrorism, including how key institutional and State actors have approached criminal justice solutions in the absence of one.

As was discussed in Module 1, the concept of "terror" is not a new one, having existed in different forms for centuries. The reasons for this are many. There are a number of possible explanations for this, some of which are explored below in relation to ongoing efforts to agree on a Comprehensive Convention.

Customary definition of terrorism

Although there is no current agreement regarding of a universal legal definition of the term, there has been some debate regarding the possible existence of an, at least partial, customary definition of terrorism. This followed the somewhat controversial judgment of the Special Tribunal for Lebanon in 2011, which found that since at least 2005, a definition of "transnational terrorism" has existed within customary international law:

As we shall see, a number of treaties, UN resolutions, and the legislative and judicial practice of States evince the formation of a general opinio juris in the international community, accompanied by a practice consistent with such opinio, to the effect that a customary rule of international law regarding the international crime of terrorism, at least in time of peace, has indeed emerged. This customary rule requires the following three key elements: (i) the perpetration of a criminal act (such as murder, kidnapping, hostage-taking, arson, and so on), or threatening such an act; (ii) the intent to spread fear among the population (which would generally entail the creation of public danger) or directly or indirectly coerce a national or international authority to take some action, or to refrain from taking it; (iii) when the act involves a transnational element. ( Interlocutory Decision, 2011, para. 85).

In reaching such a finding, the Tribunal relied primarily upon relevant United Nations policies, practices, and norms, including those of the General Assembly, as well as upon national and international jurisprudence. Furthermore, it was stated by the Tribunal that the necessary substantive (objective and subjective) elements for two other classes of terrorist criminal conduct also existed within international law: war crimes committed in the course of international or non-international armed conflict; and those acts crossing the threshold to constitute crimes against humanity, whether perpetrated during peace time or armed conflict.

While the existence (although not necessarily the interpretation) of the latter two categories of offences are non-contentious and well established within international law (including treaties and related jurisprudence), the existence of a peacetime international crime within customary international law is not widely regarded as being settled. Certainly, whenever terrorism terminology is referred to within a United Nations instrument, such as a resolution, it should not be understood as implying the existence of a customary definition. Notably, as will be seen, the General Assembly Declaration on measures to eliminate international terrorism (resolution 49/60) requires a political purpose, but the draft Comprehensive Convention does not. Meanwhile Security Council Resolution 1566 (2004) focuses on reiterating sectoral offences, does not incorporate all forms of terrorism, and does not require any special intent or motive.

The Tribunal's ruling, together with its underlying legal basis, were significantly criticized and not widely accepted, including for not meeting the necessary legal threshold tests in terms of state practice and opinio juris. As leading commentator Ben Saul noted at the time:

While there are numerous sector-specific treaties which address particular criminal means and methods used by terrorists, none of [the treaties referred to by the Special Tribunal] - individually or collectively - contains a comprehensive definition of terrorism or establishes a general international crime of transnational "terrorism". At most, specific offences in some treaties may have entered into customary law, such as aircraft hijacking or hostage taking. In the absence of a general crime of terrorism in treaty law, no parallel customary rule can arise out of those treaties. The sectoral approach was adopted precisely because states could not reach agreement on 'terrorism' as such. (Saul, 2012, p. lxxi).

Indeed, as Saul further comments, although the Tribunal sought to rely on regional instruments against terrorism as partial evidence of support for its findings, a correct reading of them in fact reveals that no agreement exists regarding a common definition of terrorism (Saul, 2012, p. lxxi), as was illustrated above. More generally, the consensus seems to be that a customary definition of terrorism is potentially evolving, but that its existence was declared prematurely by the Special Tribunal.

Implications of the absence of a universal definition of terrorism

The implications of the absence of a universal definition of terrorism for legal purposes are wide-ranging. One is that the lack of a definition may faciliate the politicization and misuse of the term "terrorism" to curb non-terrorist (or sometimes even non-criminal) activities. In turn, this can result in States, e.g., violating the rights of their own or other States' citizens, such as those of international human rights law, in the course of their counter-terrorism efforts.

Where domestic laws also suffer from ambiguity as to their full reach and meaning in terms of the activities criminalized and the implications thereof, such laws can offend the principle of legality, or nullum crimen, nulla poena sine lege. This maxim requires that a person should not face criminal trial or punishment under domestic or international criminal law except for an act that was criminalized at the time the act was allegedly committed, meaning that a criminal law cannot be applied retroactively. Of particular interest here, an inherent aspect of this legal principle is that there should be certainty in the law such that the criminalized acts and their accompanying penalties are clearly defined, without doubt or ambiguity, prior to their alleged commission.

Another significant issue has been the lack of harmonization between national and regional laws and normative standards on countering terrorism. As was discussed in Module 3, following the 9/11 terrorist attacks, the Security Council adopted Resolution 1373 (2001), which required States to take effective national legislative action as part of their global efforts to counter terrorism more effectively. Although, on the one hand, this obligated States to take legislative action, in the absence of a universally agreed definition of terrorism, the result has been a mixed legislative response and approach by Member States, sometimes with the potential to hinder rather than facilitate international cooperation.

United Nations approach

Within the United Nations system, in the absence of a universally agreed definition of the term, various terminology describing the notion of "terrorism" can be found within its outputs. These are not generally intended to suggest the existence of an agreed definition of terrorism (although, ultimately, that is one of the sought but currently elusive goals of the draft Comprehensive Convention on International Terrorism (Comprehensive Convention) discussed below). Instead, they are intended to act as guidance benchmarks to assist States in, e.g., carrying out actions requested or required by particular outputs and instruments. Various examples are considered here by way of illustration, drawing from a number of different sources.

The first example is General Assembly resolution 49/60, which seeks to criminalize a number of armed activities considered to be "terrorist" in nature:

[A]cts intended or calculated to provoke a state of terror in the general public, a group of persons or particular persons for political purposes are in any circumstance unjustifiable, whatever the considerations of a political, philosophical, ideological, racial, ethnic, religious or any other nature that may be invoked to justify them. (Para. 3).

A particular strength of this approach is that it was adopted by consensus, meaning without a vote, by the General Assembly. Therefore, it enjoys a high level of legitimacy as the outcome of the fully representational forum of the General Assembly and thus remains influential. As an output of the General Assembly, however, it is not binding in nature. This conclusion is further evidenced by the fact that it has not yet been possible to agree on a universal definition in the context of the ongoing negotiations regarding the draft Comprehensive Convention. Certainly, General Assembly resolution 49/60 recognized the need for the progressive development and codification of anti-terrorism norms (para. 12). For such reasons, though resolution 49/60 remains important and influential, including through the annual re-adoption by the General Assembly of its "measures to eliminate terrorism" resolutions, any potential law-making role remains unclear.

Another example of "terrorism" related terminology is Resolution 1566 (2004), which aimed to assist States in meeting their obligations under Security Council Resolution 1373 (2001) to take domestic legislative action. It refers to "terrorism" as:

... criminal acts, including against civilians, committed with intent to cause death or serious bodily injury, or taking of hostages, with the purpose to provoke a state of terror in the general public or in a group of persons or particular persons, intimidate a population or compel a government or an international organization to do or to abstain from doing any act, which constitute offences within the scope of and as defined in the international conventions and protocols relating to terrorism, are under no circumstances justifiable by considerations of a political, philosophical, ideological, racial, ethnic, religious or other similar nature, and calls upon all States to prevent such acts and, if not prevented, to ensure that such acts are punished by penalties consistent with their grave nature. (Para. 3).

Adopted by the Security Council under Chapter VII, potentially the argument could be made that the resolution is legally binding on all Member States. In practice, however, any such suggestion - e.g., that an accompanying obligation exists to accept and implement a particular definition of "terrorism" - is likely to be strongly rejected by States. (See further Security Council, Counter-Terrorism Committee survey S/2016/50). That said, Resolution 1566 has played an important role in influencing and assisting in the harmonization of some definitions of terrorism within national laws. It has also been significant in terms of closing gaps regarding such offences within domestic criminal frameworks (Young, 2006) and forms part of the body of norms influencing how terrorism offences are categorized and dealt with today.

A further approach (considered below) exists within the context of the draft Comprehensive Convention article 2. Once again, some important differences can be discerned between the terminology used here compared with those employed by the General Assembly and Security Council. On the one hand, it is clearer in scope than the General Assembly's Declaration of 1994 and Security Council Resolution 1566 in expressly referring to property damage as part of the criminal act. On the other hand, it is less extensive in articulating the grounds which will never justify terrorist acts. This may be attributable, at least in part, to the fact that as a potentially legally binding treaty the draft Comprehensive Convention is subject to a more rigorous process of negotiation and scrutiny compared with resolutions.

More generally, as is examined in some detail below, most universal anti-terrorism instruments do not contain a definition of terrorism due to its accompanying political sensitivities. One notable exception though is the example discussed here of article 2 of the International Convention for the Suppression of the Financing of Terrorism of 1999. This provision criminalizes terrorism financing in the following terms:

1. Any person commits an offence within the meaning of this Convention if that person by any means, directly or indirectly, unlawfully and wilfully, provides or collects funds with the intention that they should be used or in the knowledge that they are to be used, in full or in part, in order to carry out:

(a) An act which constitutes an offence within the scope of and as defined in one of the treaties listed in the annex; or

(b) Any other act intended to cause death or serious bodily injury to a civilian, or to any other person not taking an active part in the hostilities in a situation of armed conflict, when the purpose of such act, by its nature or context, is to intimidate a population, or to compel a government or an international organization to do or to abstain from doing any act.

This text was reaffirmed by Security Council Resolution 1373 (2001), which called upon all States to "[b]ecome parties as soon as possible to the relevant international conventions and protocols relating to terrorism, including the International Convention for the Suppression of the Financing of Terrorism of 9 December 1999." (Para. 3(d)).

Regional and national approaches

In addition to States, many regional organizations have counter-terrorism instruments (see Module 5), which contain their own definitions of terrorism that reflect regional priorities. The same issues, however, have arisen in relation to many of these definitions as for national approaches, namely that their inter-regional divergence in approach has the potential to impede rather than facilitate wider international cooperation among their membership. In addition, important differences in definitional approaches can arise between regional organizations and those of their individual Member States. In turn, these can pose conflict of law challenges regarding which rule should prevail - treaty law or domestic law - in the event of inconsistencies between two sets of rules which cannot be reconciled. (See further Charter of the United Nations 1945, article 103; Vienna Convention on the Law of Treaties 1969, article 30). Generally, including under the principle of incorporation discussed in Module 3, domestic law will prevail in practice, including for constitutional reasons.

The following table outlines the principal definitional approaches of those regional organizations examined in Module 5. None are suggested to represent a universally agreed definition of terrorism within the United Nations system.

Regional definitional approaches

(1) Armed national liberation or self-determination struggle exemption

This approach distinguishes between what is regarded as constituting unlawful, criminal acts of terrorists, and the perpetration of violence committed as part of what is regarded to be a legitimate struggle against any occupation, aggression, or domination by foreign forces, making the latter exempt from criminal proceedings, for example:

  • The Arab Convention for the Suppression of Terrorism (adopted 22 April 1998, entered into force 7 May 1999), article 2(a) provides that:
    • "All cases of struggle by whatever means, including armed struggle, against foreign occupation and aggression for liberation and self-determination, in accordance with the principles of international law, shall not be regarded as an offence. This provision shall not apply to any act prejudicing the territorial integrity of any Arab State."


  • Organization of the Islamic Conference (OIC) Convention on Combating International Terrorism (adopted 1 July 1999, entered into force 7 November 2002), article 2(a).
  • Organization of African Unity (OAU) Convention on the Prevention and Combating of Terrorism (adopted 14 July 1999, entered into force 6 December 2003), article 3(1).

These regional instruments adopt different approaches regarding the criminal elements necessary to establish terrorism. See further:

  • Arab Convention for the Suppression of Terrorism, article 1(2).
  • Organization of the Islamic Conference Convention on Combating International Terrorism, article 1(2).
  • Organization of African Unity Convention on the Prevention and Combating of Terrorism, article 1(3).

(2) Reference to sectoral conventions against terrorism

The approach of most other regional conventions has been to cross-refer to the approach of the sectoral conventions against terrorism, which do not define terrorism but instead refer to the criminal elements of particular offences. Regional conventions falling into this category are:

  • Council of Europe Convention on the Prevention of Terrorism (adopted 16 May 2005, entered into force 1 June 2007), article 1(1).
    • Article 1(1): For the purposes of this Convention, "terrorist offence" means any of the offences within the scope of and as defined in one of the treaties listed in the Appendix (i.e. all of the sectoral conventions).
    • Note by Secretariat: "The activities of armed forces during an armed conflict, as those terms are understood under international humanitarian law, which are governed by that law, are not governed by this Convention, and the activities undertaken by military forces of a Party in the exercise of their official duties, inasmuch as they are governed by other rules of international law, are not governed by this Convention."


  • Inter-American Convention against Terrorism (adopted 3 June 2002, entered into effect 6 July 2003), article 2(1).
  • South Asia Association for Regional Cooperation Regional Convention on Suppression of Terrorism (adopted 4 November 1987), article 1.
  • Association of Southeast Asian Nations Convention on Counter Terrorism (adopted 13 January 2007, entered into force 27 May 2011), article II.

(3) Hybrid approach

A hybrid approach is adopted by the Shanghai Convention on Combatting Terrorism, Separatism and Extremism (adopted 15 June 2001, entered into effect 29 March 2003). Article 1(a) cross refers to existing anti-sectoral treaties. In article 1(b), however, a definition is also given, reflecting regional priorities:

"[A]ny other act intended to cause death or serious bodily injury to a civilian, or any other person not taking an active part in the hostilities in a situation of armed conflict or to cause major damage to any material facility, as well as to organize, plan, aid and abet such act, when the purpose of such act, by its nature or context, is to intimidate population, violate public security or compel public authorities or an international organization to do or to abstain from doing any act, and prosecuted in accordance with the national laws of the Parties."

(4) Adoption of regional definition reflecting international norms

This has been the approach of the European Union which defined its own approach:

  • European Union, Council of the European Union, Council Framework Decision of 13 June 2002 on combating terrorism, article 1.

As has been mentioned, there are numerous examples, where the term terrorism has been defined at a national level in the absence of agreement regarding a universal definition. Some examples of different national approaches are included in the suggested reading for this module.

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