Terrorism: interpretation.

(1)In this Act “terrorism” means the use or threat of action where—
(a)the action falls within subsection (2),
(b)the use or threat is designed to influence the government [F1or an international governmental organisation] or to intimidate the public or a section of the public, and
(c)the use or threat is made for the purpose of advancing a political, religious [F2, racial] or ideological cause.
(2)Action falls within this subsection if it—
(a)involves serious violence against a person,
(b)involves serious damage to property,
(c)endangers a person’s life, other than that of the person committing the action,
(d)creates a serious risk to the health or safety of the public or a section of the public, or
(e)is designed seriously to interfere with or seriously to disrupt an electronic system.
(3)The use or threat of action falling within subsection (2) which involves the use of firearms or explosives is terrorism whether or not subsection (1)(b) is satisfied.
(4)In this section—
(a)“action” includes action outside the United Kingdom,
(b)a reference to any person or to property is a reference to any person, or to property, wherever situated,
(c)a reference to the public includes a reference to the public of a country other than the United Kingdom, and
(d)“the government” means the government of the United Kingdom, of a Part of the United Kingdom or of a country other than the United Kingdom.
(5)In this Act a reference to action taken for the purposes of terrorism includes a reference to action taken for the benefit of a proscribed organisation.

http://www.legislation.gov.uk/ukpga/2000/11/section/1