Terrorism is political, ideological or religious violence by non-state actors.

The perpetrated acts of violence have the intention of creating fear and terror for political, ideological or religious goals, and target the safety of civilians and neutral military personnel (non-combatants). Although precise criteria exist, there exist over 100 definitions of the term and there is no international criminal law definition.

In January 2015, the Paris attack against the Charlie Hebdo newspaper personnel made the fight against terror an essential priority in many countries of which Britain. On a global scale, the EU's has made the alliance against terrorism one of its central goals. .

Although it is sometimes identified with jihadism or Islamism, and with Islamist groups such as ISIS (Islamic State of Iraq and the Levant) and Boko Haram, it has been practiced by all sorts of actors, by right-wing and left-wing political parties, nationalist or religious groups, and governments themselves. The goals of the group perpetrating it are partly achieved by the creation of fear.

The ones who are labeled as terrorists by their opponents usually consider themselves in other terms, such as "freedom fighter", "rebel", "guerilla" or similar words in other languages, such as "Jihadi" in Arabic.

The different types of terrorism used have evolved and varied over time. Classical terrorism uses a direct approach and aims a particular target and few casualties: it is used for political purposes. Modern terrorism uses a more indirect approach, indiscriminate methods and aims at inflicting the most casualties possible. Postmodern terrorism changes the reality of terrorism itself and is aimed at symbols of the enemy. The latter is the type of terrorism that is upheld by religious groups such as ISIS. New instruments, such as social media, are used to promote this type of terrorism.

Countries and international organizations such as the United Nations hold lists of terrorist groups, which can vary from one list to another. The Provisional Irish Republican Army (IRA), for instance, is still considered a terrorist organization in the United Kingdom, but is considered an unlawful organization in the Republic of Ireland.