“Any activity that involves an act that is dangerous to human life or potentially destructive of critical infrastructure or key resources; and must also 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.”

Source: Inside Terrorism, Bruce Hoffman
FBI definition of terrorism:
“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.”

Source: http://www.nij.gov/topics/crime/terrorism/pages/welcome.aspx

About the Author's Definition of Terrorism:

Defining terrorism can be tricky seeing that within the U.S. government there are numerous definitions depending on the department.  Each definition of terrorism is shaped by the objectives of a particular department, being as vague or as specific as they feel the need. However, one can look to other sources for an overarching definition of terrorism that is accepted in our modern society. One author that provides a definition of terrorism that is widely accepted is Bruce Hoffman.  He emphasizes the notion that terrorism by definition is inherently political as a concept and must be examined with this characteristic in mind.   He further explains that terrorism is about power and the use of power to achieve political change.  He thus defines terrorism as: violence or the threat of violence used and directed in pursuit of, or in service of, a political aim.

Source: Inside Terrorism pp. 3-4, Bruce Hoffman