Terrorism is not universally defined many government agencies have they're own definitions of terrorism, while similar they are each unique.  FEMA uses the following definition of terrorism:

"Terrorism is the use of force or violence against persons or property in violation of the criminal laws of the United States for purposes of intimidation, coercion, or ransom. Terrorists often use threats to:

•   Create fear among the public.

•   Try to convince citizens that their government is powerless to prevent terrorism.

18 U.S.C. § 2331 defines "international terrorism" and "domestic terrorism" for purposes of Chapter 113B of the Code, entitled "Terrorism”:

"International terrorism" means activities with the following three characteristics:

• Involve violent acts or acts dangerous to human life that violate federal or state law;

• 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

• Occur primarily outside the territorial jurisdiction of the U.S., or transcend national boundaries in terms of the means by which they are accomplished, the persons they appear intended to intimidate or coerce, or the locale in which their perpetrators operate or seek asylum.*

"Domestic terrorism" means activities with the following three characteristics:

• Involve acts dangerous to human life that violate federal or state law;

• Appear 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

• Occur primarily within the territorial jurisdiction of the U.S.

18 U.S.C. § 2332b defines the term "federal crime of terrorism" as an offense that:

• Is calculated to influence or affect the conduct of government by intimidation or coercion, or to retaliate against government conduct; and

• Is a violation of one of several listed statutes, including § 930(c) (relating to killing or attempted killing during an attack on a federal facility with a dangerous weapon); and § 1114 (relating to killing or attempted killing of officers and employees of the U.S.).

It is helpful to know how scholars in the study of terrorism define it. 

Jessica stern author of "Terror in the Name of God" defines terrorism as such: "...terrorism will be defined as an act or threat of violence against noncombatants with the objective of exacting revenge, intimidating, or otherwise influencing an audience. This definition avoids limiting perpetrator or purpose.  It allows for a range of possible actors (states or their surrogates, international groups, or a single individual) and all putative goals (political, religious, or economic)" (Stern XX).

Bruce Hoffman author of "Inside Terrorism" as follows:

"By distinguishing terrorists from other types of criminals and irregular fighters and terrorism from other forms of crime and irregular warfare, we come to appreciate that terrorism is:

-ineluctably political in aims and motives

-violent -- or, equally important, threatens violence

-designed to have far-reaching psychological repercussions beyond the immediate victim or

target

-conducted either by an organization with an identifiable chain of command or conspiratorial cell structure (whose members wear no uniform or identifying insignia) or by individuals or a small collection of individuals directly influenced, motivated, or inspired by the ideological aims or example of some existent terrorist movement and/or its leaders

-perpetrated by a subnational group or nonstate entity.