The Terrorism Act 2000 (c. 11) is the first of a number of general Terrorism Acts passed by the Parliament of the United Kingdom. It superseded and repealed the Prevention of Terrorism (Temporary Provisions) Act 1989 and the Northern Ireland (Emergency Provisions) Act 1996. It also replaced parts of the Criminal Justice (Terrorism and Conspiracy) Act 1998. The powers it provides the police have been controversial, leading to noted cases of alleged abuse, and to legal challenges in British and European courts. The stop-and-search powers under section 44 of the Act have been ruled illegal by the European Court of Human Rights.

Terrorism is defined, in the first section of the act, as follows:

Sections (2)(b) and (e) have been criticised as falling well outside the scope of what is generally understood to be the definition of terrorism, i.e. acts that require life-threatening violence.

Prior to this, terrorism was defined in an Act as a footnote, such as Reinsurance (Acts of Terrorism) Act 1993 (c. 18) section 2(2):

"acts of terrorism" means acts of persons acting on behalf of, or in connection with, any organisation which carries out activities directed towards the overthrowing or influencing, by force or violence, of Her Majesty's government in the United Kingdom or any other government de jure or de facto.

and Prevention of Terrorism (Temporary Provisions) Act 1989 (c. 4) section 20(1):

In this Act "terrorism" means the use of violence for political ends, and includes any use of violence for the purpose of putting the public or any section of the public in fear.

In the Northern Ireland (Emergency Provisions) Act 1996, the criminal offences referred to as terrorism are provided as an exhaustive list of over 70 items.