Official definitions determine counter-terrorism policy and are often developed to serve it.

Most government definitions outline the following key criteria: target, objective, motive, perpetrator, and legitimacy or legality of the act.

Terrorism is also often recognizable by a following statement from the perpetrators.

  Violence - According to Walter Laqueur of the Center for Strategic and International Studies, "the only general characteristic of terrorism generally agreed upon is that terrorism involves violence and the threat of violence." However, the criterion of violence alone does not produce a useful definition, as it includes many acts not usually considered terrorism: war, riot, organized crime, or even a simple assault. Property destruction that does not endanger life is not usually considered a violent crime, but some have described property destruction by the Earth Liberation Front and Animal Liberation Front as violence and terrorism; see eco-terrorism.

  Psychological impact and fear - The attack was carried out in such a way as to maximize the severity and length of the psychological impact. Each act of terrorism is a "performance," devised to have an impact on many large audiences. Terrorists also attack national symbols to show their power and to shake the foundation of the country or society they are opposed to. This may negatively affect a government's legitimacy, while increasing the legitimacy of the given terrorist organization and/or ideology behind a terrorist act.

  Perpetrated for a Political Goal - Something all terrorist attacks have in common is their perpetration for a political purpose. Terrorism is a political tactic, not unlike letter writing or protesting, that is used by activists when they believe no other means will effect the kind of change they desire. The change is desired so badly that failure is seen as a worse outcome than the deaths of civilians. This is often where the interrelationship between terrorism and religion occurs. When a political struggle is integrated into the framework of a religious or "cosmic" struggle, such as over the control of an ancestral homeland or holy site such as Israel and Jerusalem, failing in the political goal (nationalism) becomes equated with spiritual failure, which, for the highly committed, is worse than their own death or the deaths of innocent civilians.

  Deliberate targeting of non-combatants - It is commonly held that the distinctive nature of terrorism lies in its intentional and specific selection of civilians as direct targets. Specifically, the criminal intent is shown when babies, children, mothers, and the elderly are murdered, or injured, and put in harms way. Much of the time, the victims of terrorism are targeted not because they are threats, but because they are specific "symbols, tools, animals or corrupt beings" that tie into a specific view of the world that the terrorist possess. Their suffering accomplishes the terrorists' goals of instilling fear, getting a message out to an audience, or otherwise accomplishing their often radical religious and political ends.

  Disguise - Terrorists almost invariably pretend to be non-combatants, hide among non-combatants, fight from in the midst of non-combatants, and when they can, strive to mislead and provoke the government soldiers into attacking the wrong people, that the government may be blamed for it. When an enemy is identifiable as a combatant, the word terrorism is rarely used. Mass executions of hostages, as by the Nazi military forces in the Second World War, certainly constituted crimes against humanity but are not commonly called terrorism.

  Unlawfulness or illegitimacy - Some official (notably government) definitions of terrorism add a criterion of illegitimacy or unlawfulness  to distinguish between actions authorized by a "legitimate" government (and thus "lawful") and those of other actors, including individuals and small groups. Using this criterion, actions that would otherwise qualify as terrorism would not be considered terrorism if they were government sanctioned. For example, firebombing a city, which is designed to affect civilian support for a cause, would not be considered terrorism if it were authorized by a "legitimate" government. This criterion is inherently problematic and is not universally accepted, because: it denies the existence of state terrorism; the same act may or may not be classed as terrorism depending on whether its sponsorship is traced to a "legitimate" government; "legitimacy" and "lawfulness" are subjective, depending on the perspective of one government or another; and it diverges from the historically accepted meaning and origin of the term. For these reasons this criterion is not universally accepted. Most dictionary definitions of the term do not include this criterion.

  Todays Terrorism is classified into 6 categories:

Civil Disorders - A form of collective violence interfering with the peace, security, and normal functioning of the community.

Political Terrorism - Violent criminal behaviour designed primarily to generate fear in the community, or substantial segment of it, for political purposes.

Non-Political Terrorism - Terrorism that is not aimed at political purposes but which exhibits "conscious design to create and maintain high degree of fear for coercive purposes, but the end is individual or collective gain rather than the achievement of a political objective."

Quasi-Terrorism - The activities incidental to the commission of crimes of violence that are similar in form and method to genuine terrorism but which nevertheless lack its essential ingredient. It is not the main purpose of the quasi-terrorists to induce terror in the immediate victim as in the case of genuine terrorism, but the quasi-terrorist uses the modalities and techniques of the genuine terrorist and produces similar consequences and reaction. For example, the fleeing felon who takes hostages is a quasi-terrorist, whose methods are similar to those of the genuine terrorist but whose purposes are quite different.

Limited Political Terrorism - Genuine political terrorism is characterized by a revolutionary approach; limited political terrorism refers to "acts of terrorism which are committed for ideological or political motives but which are not part of a concerted campaign to capture control of the State.