From Wikipedia:
Terrorism is the systematic use of terror especially as a means of coercion. There is no internationally agreed definition of terrorism. Most common definitions of terrorism include only those acts which are intended to create fear (terror), are perpetrated for an ideological goal (as opposed to a lone attack), and deliberately target or disregard the safety of non-combatants.
Some definitions also include acts of unlawful violence and war. The history of terrorist organizations suggests that they do not select terrorism for its political effectiveness. Individual terrorists tend to be motivated more by a desire for social solidarity with other members of their organization than by political platforms or strategic objectives, which are often murky and undefined. The word "terrorism" is politically and emotionally charged, and this greatly compounds the difficulty of providing a precise definition. One 1988 study by the US Army found that over 100 definitions of the word "terrorism" have been used. A person who practices terrorism is a terrorist. The concept of terrorism is itself controversial because it is often used by states to delegitimize political opponents, and thus legitimize the state's own use of terror against those opponents.
Terrorism has been used by a broad array of political organizations in furthering their objectives; both right-wing and left-wing political parties, nationalistic, and religious groups, revolutionaries and ruling governments. The presence of non-state actors in widespread armed conflict has created controversy regarding the application of the laws of war.
While acts of terrorism are criminal acts as per the United Nations Security Council Resolution 1373 and domestic jurisprudence of almost all countries in the world, terrorism refers to a phenomenon including the actual acts, the perpetrators of acts of terrorism themselves and their motives. There is disagreement on definitions of terrorism.