The definition of ''Terrorism'' can be found in most dictionary as ''the use of violence and threats to intimidate or coerce, esp. for political purposes.'' 

But in fact there has never been made an global definition of what an act of terrorism is.
The word first occured in 1795, 30 January. It was wrote in The Times (a english newpaper), the excerpt reads: ''There exists more than one system to overthrow our liberty. Fantacism has raised every passion; Royalism has not yet given up its hopes, and Terrorism feels bloder than ever''.
The term ''terrorism" come from the Latin terrere ''to scare or to frighten", and it was created through the French word terrorisme, which is often associated with ''regime de la terreur'' (read explanation written in Living With Terrorism).

The thing that terrorism doesn't have a global definition creates different view points in when people shall be judged (and how hard they are to be punished).
United Nations (the closest we comes to a organization which involves
all countries) haven't accepted a definition of terrorism, but they uses another definition when they need to use it, this definition of terrorism is written by terrorism expert Alex P. Schmid. It says ''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 assasination - the direct targets of violence are not the main targets. The immediate human victims of violence are generally chosen randomly (targets of opportunity) or slectively (representative or symbolic targets) from a target population, and serve as message generators. Threat- and violence-based communication processes between terrorist (organization), (imperiled) victims, and main targets are used to manipulate the main target (audience(s)), turning it into a target of terrror, a target of demands, or target of attention depending on whether intimidation, coercion, or propaganda is primaly sought.'' Closer to a real definition we can't get, but you can see lots of more definition on various sites.

But we can conclude that terrorism can't be explained as so, because there are many different meanings on what terrorism is.

Laszlo Joergensen

More information can be found Here or Here