> "terrorism - the systematic use of terror especially as a means of coercion."
Interesting side effect of this definition is that because the main effect of terrorism is not material or human casualties, but terror, it's usually less damaging way of waging a war than conventional warfare. There are typically more civilian casualties in conventional warfare where civilians are just collateral than terrorist campaigns that target civilians.
Six Rather Unusual Propositions about Terrorism
https://politicalscience.osu.edu/faculty/jmueller/6PROPS.PDF
> The costs of terrorism very often come mostly from the fear and consequent reaction
(or overreaction) it characteristically inspires (qualities stoked by the terrorism indus-
try), not from its direct effects which are usually comparatively limited. Therefore,
policies designed to deal with terrorism should focus more on reducing fear and anxiety
as inexpensively as possible than on objectively reducing the rather limited dangers
terrorism is likely actually to pose. Doing nothing (or at least refraining from
overreacting) after a terrorist attack is not necessarily unacceptable, and, despite
U. S. overreaction, the campaign against terror is generally going rather well because,
no matter how much they might disagree on other issues (most notably on war in Iraq),
there is a compelling incentive for states to cooperate to deal with a common problem
‘terrorism industry, an entity that includes not only various risk entrepreneurs and bureaucrats, but also
most of the media and nearly all politicians.