Additionally, the 9/11 attack was the initiation of a war according to the rhetoric of the powers that be. Here is a common definition of terrorism.
Definitions of Terrorism
Much of the early work in terrorism research centered upon various definitions (see the discussions in Cooper, 2001; Gibbs, 1989; Hoffman, 1999; Jenkins, 2001; Ruby, 2002; Schmid and Jongman, 1988; Senechal de la Roche, 1996, 2001), but as Jenkins (2001) notes, a consensus seems to be emerging on the definition of terrorism. For example, academic researchers Walter Enders and Todd Sandler (2002b) argue terrorism involves a focus upon underlying political, social, or religious motives, as its violence is separable from crime, personal vengeance, or the act of someone mentally deranged. The act itself seems to involve attempts at influencing an audience, which is often not that of the victims themselves. Terrorism is also most often directed toward noncombatants or civilians and is ‘random,’ so that everyone feels at risk. For such terrorists: Terrorism is the premeditated use or threat of use of extranormal violence or brutality by subnational groups to obtain a political, religious, or ideological objective through intimidation of a huge audience, usually not directly involved
with the policy making that the terrorists seek to influence. (Enders and Sandler, 2002b: 145–6) Government institutions, such as the US Department of State, define terrorism somewhat similarly, as ‘politically motivated violence perpetrated
against noncombatant targets by subnational groups or clandestine agents, usually intended to influence an audience’ (Ruby, 2002: 10). And, interestingly enough, this is quite similar to Chomsky’s (2001: 19) definition: ‘Terrorism is the use of coercive means aimed at populations in an effort to achieve political, religious, or other aims.’ Similar to these is Stern’s (1999: 30) definition of terrorism as ‘an act or threat of violence against non-combatants, with the objective of intimidating or otherwise influencing an audience or audiences.’ The point is that academic researchers, government agencies, and critics of American foreign policy increasingly seem to agree about the essence of terrorism. This conception leaves open questions of motivation and ideology, which is important for researchers who wish to focus comparatively upon different historical periods and must be flexible enough to include the ideology of anarchists and social revolutionaries at the end of the 19th century and fundamental Islamic beliefs in the early 21st century. It also leaves open the question of
whether the violence is performed by the state or subnational groups.
Bergesen & Han. New Directions for Terrorism Research.
International Journal of Comparative Sociology. 46 (12): 133.
As you can see, many early definitions focused on terrorism occurring at the subnational level, but I think that is a head-in-the-sand thought process to alleviate responsibility of those unwilling to challenge the state on such issues.