Here is the definition of terrorism for domestic security in Section 802 of the Patriot Act:
SEC. 802. DEFINITION OF DOMESTIC TERRORISM.
(a) DOMESTIC TERRORISM DEFINED.—Section 2331 of title 18,
United States Code, is amended—
(1) in paragraph (1)(B)(iii), by striking ‘‘by assassination
or kidnapping’’ and inserting ‘‘by mass destruction, assassination,
or kidnapping’’;
(2) in paragraph (3), by striking ‘‘and’’;
(3) in paragraph (4), by striking the period at the end
and inserting ‘‘; and’’; and
(4) by adding at the end the following:
‘‘(5) the term ‘domestic terrorism’ means activities that—
‘‘(A) involve acts dangerous to human life that are
a violation of the criminal laws of the United States or
of any State;
‘‘(B) appear to be intended—
‘‘(i) to intimidate or coerce a civilian population;
‘‘(ii) to influence the policy of a government by
intimidation or coercion; or
‘‘(iii) to affect the conduct of a government by
mass destruction, assassination, or kidnapping; and
‘‘(C) occur primarily within the territorial jurisdiction
of the United States.’’.
(b) CONFORMING AMENDMENT.—Section 3077(1) of title 18,
United States Code, is amended to read as follows:
‘‘(1) ‘act of terrorism’ means an act of domestic or international
terrorism as defined in section 2331;’’.
http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbna... Here is a part of the ACLU's comment on that definition:
Section 802 of the USA PATRIOT Act (Pub. L. No. 107-52) expanded the definition of terrorism to cover ""domestic,"" as opposed to international, terrorism. A person engages in domestic terrorism if they do an act ""dangerous to human life"" that is a violation of the criminal laws of a state or the United States, if the act appears to be intended to: (i) intimidate or coerce a civilian population; (ii) influence the policy of a government by intimidation or coercion; or (iii) to affect the conduct of a government by mass destruction, assassination or kidnapping. Additionally, the acts have to occur primarily within the territorial jurisdiction of the United States and if they do not, may be regarded as international terrorism.
Section 802 does not create a new crime of domestic terrorism. However, it does expand the type of conduct that the government can investigate when it is investigating ""terrorism."" The USA PATRIOT Act expanded governmental powers to investigate terrorism, and some of these powers are applicable to domestic terrorism.
The definition of domestic terrorism is broad enough to encompass the activities of several prominent activist campaigns and organizations. Greenpeace, Operation Rescue, Vieques Island and WTO protesters and the Environmental Liberation Front have all recently engaged in activities that could subject them to being investigated as engaging in domestic terrorism.
. . . .
http://www.aclu.org/national-security/how-usa-patriot-a... International terrorism is separately defined. Here is that definition under the U.S. Code:
United States Law Code – the law that governs the entire country – contains a definition of terrorism embedded in its requirement that Annual Country reports on Terrorism be submitted by the Secretary of State to Congress every year. (From U.S. Code Title 22, Ch.38, Para. 2656f(d)
(d) Definitions
As used in this section—
(1) the term “international terrorism” means terrorism involving citizens or the territory of more than 1 country;
(2) the term “terrorism” means premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents;
(3) the term “terrorist group” means any group, or which has significant subgroups which practice, international terrorism;
(4) the terms “territory” and “territory of the country” mean the land, waters, and airspace of the country; and
(5) the terms “terrorist sanctuary” and “sanctuary” mean an area in the territory of the country—
(A) that is used by a terrorist or terrorist organization—
(i) to carry out terrorist activities, including training, fundraising, financing, and recruitment; or
(ii) as a transit point; and
(B) the government of which expressly consents to, or with knowledge, allows, tolerates, or disregards such use of its territory and is not subject to a determination under—
(i) section 2405(j)(1)(A) of the Appendix to title 50;
(ii) section 2371 (a) of this title; or
(iii) section 2780 (d) of this title.
http://terrorism.about.com/od/whatisterroris1/ss/Define...