Terrorism can be defined as the systematic belief in the political, religious, or ideological efficacy of producing fear by attacking or threatening to attack unsuspecting or defenseless populations, usually civilians, usually by surprise. Terrorist attacks are the desperate acts of those who feel themselves to be otherwise powerless. Terrorism is self-righteous, absolutist, and exclusivist. Its adherents are unwilling or unable to negotiate with their perceived enemies, or are prevented by political, social, or economic circumstances from doing so.

The terms “terrorism” and “terrorist” came into the language in the 1790s when British journalists, politicians, orators, and historians used them to describe the Jacobins and other particularly violent French revolutionaries. The terms have evolved since then and now typically refer to furtive acts by unknown underground perpetrators, not by heads of state or public figures such as Caligula, Ivan the Terrible, Maximilien Robespierre, or Joseph Stalin.

Terrorism as we now understand it was not possible until the invention of gunpowder and subsequent explosives and incendiaries. Before that, political assassinations, gigantic arsons, large-scale massacres, and other horrendously terrifying acts certainly occurred, but small cadres of insignificant citizens generally lacked the means to actualise sudden massive destruction by stealth. Roger Bacon's invention of gunpowder in the thirteenth century and the introduction of firearms in Europe in the fourteenth enabled weaklings to outmatch and regularly defeat, for the first time in history, mighty warriors armed with sword or spear. Davids proliferated and all Goliaths became vulnerable.