Title 22 of the United States Code, Section 2656f(d). That statute contains the following definitions:

The term “terrorism” means premeditated, politically motivated violence perpetrated against noncombatant (1) targets by subnational groups or clandestine agents, usually intended to influence an audience.
The term “international terrorism” means terrorism involving citizens or the territory of more than one country.
The term “terrorist group” means any group practicing, or that has significant subgroups that practice, international terrorism.
The U.S. Government has employed this definition of terrorism for statistical and analytical purposes since 1983.

Online Etymology Dictionary:

1795, in specific sense of “government intimidation during the Reign of Terror in France” (1793-July 1794), from Fr. terrorisme (1798), from L. terror.

“If the basis of a popular government in peacetime is virtue, its basis in a time of revolution is virtue and terror — virtue, without which terror would be barbaric; and terror, without which virtue would be impotent.” [Robespierre, speech in Fr. National Convention, 1794]

General sense of “systematic use of terror as a policy” is first recorded in Eng. 1798. Terrorize “coerce or deter by terror” first recorded 1823. Terrorist in the modern sense dates to 1947, especially in reference to Jewish tactics against the British in Palestine — earlier it was used of extremist revolutionaries in Russia (1866); and Jacobins during the French Revolution (1795) — from Fr. terroriste. The tendency of one party’s terrorist to be another’s guerilla or freedom fighter was noted in ref. to the British action in Cyprus (1956) and the war in Rhodesia (1973). The word terrorist has been applied, at least retroactively, to the Maquis resistance in occupied France in World War II (e.g. in the “Spectator,” Oct. 20, 1979).

WordNet:

“The calculated use of violence (or the threat of violence) against civilians in order to attain goals that are political or religious or ideological in nature; this is done through intimidation or coercion or instilling fear.”