J.R's GLobal Security Resource uses the definition set forth by the U.S Federal Bureau of Investigation:

Terrorism is the unlawful use of force or violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives. 
--FBI Definition 

"All criminal acts directed against a State and intended or calculated to create a state of terror in the minds of particular persons or a group of persons or the general public".

2. UN Resolution language (1999):

"1. Strongly condemns all acts, methods and practices of terrorism as criminal and unjustifiable, wherever and by whomsoever committed;

2. Reiterates that criminal acts intended or calculated to provoke a state of terror in the general public, a group of persons or particular persons for political purposes are in any circumstance unjustifiable, whatever the considerations of a political, philosophical, ideological, racial, ethnic, religious or other nature that may be invoked to justify them". (GA Res. 51/210 Measures to eliminate international terrorism)

3. Short legal definition proposed by A. P. Schmid to United Nations Crime Branch (1992):

Act of Terrorism = Peacetime Equivalent of War Crime

4. Academic Consensus Definition: 

"Terrorism is an anxiety-inspiring method of repeated violent action, employed by (semi-) clandestine individual, group or state actors, for idiosyncratic, criminal or political reasons, whereby - in contrast to assassination - the direct targets of violence are not the main targets. The immediate human victims of violence are generally chosen randomly (targets of opportunity) or selectively (representative or symbolic targets) from a target population, and serve as message generators. Threat- and violence-based communication processes between terrorist (organization), (imperilled) victims, and main targets are used to manipulate the main target (audience(s)), turning it into a target of terror, a target of demands, or a target of attention, depending on whether intimidation, coercion, or propaganda is primarily sought" (Schmid, 1988).

Source:ODCCP 2001

The term "terrorism" means premeditated, politically motivated violence perpetrated against non-combatant targets by sub-national 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.

i The U.S. Government has interpreted "noncombatant" to include, in addition to civilians, military personnel who at the time of the incident are unarmed or not on duty. Similarly, the U.S. Government considers attacks on military installations or on armed military personnel when a state of military hostilities does not exist at the site to be terrorist attacks.
ii In this context, sub-national means a grouping not recognized as a nation-state. This includes groups like the Provisional Wing of the Irish Republican Army, HAMAS (Islamic Resistance Movement) or Kahane Chai.
iii An example would be the attacks on dissidents carried out by secret agents of the Iranian government.
22 U.S.C. § 2656f(d)

"(ii)TERRORIST ACTIVITY DEFINED--As used in this Act, the term "terrorist activity" means any activity which is unlawful under the laws of the place where it is committed (or which, if committed in the United States, would be unlawful under the laws of the United States or any State) and which involves any of the following:" 

"(I) The highjacking or sabotage of any conveyance (including an aircraft, vessel, or vehicle)."

"(II) The seizing or detaining, and threatening to kill, injure, or continue to detain, another individual in order compel a third person (including a governmental organization) to do or abstain from doing any act as an explicit or implicit condition for the release of the individual seized or detained."

"(III) A violent attack upon an internationally protected person (as defined in section 1116(b)(4) of title 18, United States code) or upon the liberty of such a person."

"(IV) An assassination."

"(V) The use of any--"

"(a) biological agent, chemical agent, or nuclear weapon or device," or
"(b) explosive or firearm (other than for mere personal monetary gain), with intent to endanger, directly or indirectly, the safety of one or more individuals or to cause substantial damage to property."
"(VI) A threat, attempt, or conspiracy to do any of the foregoing." 

"(iii) ENGAGE IN TERRORIST ACTIVITY DEFINED.-As used in the Act, the term "engage in terrorist activity" means to commit, in an individual capacity or as a member of an organization, an act of terrorist activity or an act which the actor knows, or reasonably should know, affords material support to any individual, organization, or government in conducting a terrorist activity at any time, including any of the following acts:" 
 

"(I) The preparation or planning of a terrorist activity."
"(II) the gathering of information on potential targets for terrorist activity."
"(III) The providing of any type of material support, including a safe house, transportation, communication, funds, false identification, weapons, explosives, or training, to any individual the actor knows or has reason to believe has committed or plans to commit a terrorist activity."
"(IV) The soliciting of funds or other things of value for terrorist activity of for any terrorist organization."
"(V) The solicitation of any individual for membership in a terrorist organization, terrorist government, or to engage in a terrorist activity."

This was taken from Section 212 (a)(3)(B) of the Immigration and Nationality Act.