It is a notion as old as human history, a cold-eyed calculation made by zealots down the ages: that the taking of innocent lives, or the threat to do so, can serve as a brutally effective means of advancing a cause.
Still, the modern age of mass terrorism — culminating with Tuesday's devastating kamikaze-style attacks against some of the greatest icons of American prestige and power — dates back only a little more than three decades.
"You look at the ancient Greeks, at Roman history, at biblical accounts — all the elements are there," said Ariel Merari, a terrorism expert at Tel Aviv University. "These are acts that we as a species have always been capable of, and we as a species have carried out. But as an international political phenomenon, it's relatively new."
The late 1960s ushered in a wave of commercial airline hijackings, the first of them carried out in 1968 by a radical Palestinian faction, with many others to follow in the name of what was then the relatively little-known Palestinian cause.
With success — in the form of world attention — a host of imitators sprang up. Left-wing and nationalist groups proliferated in Western Europe and Latin America in the 1970s and 1980s, many of them employing terror as a tactic. The arena widened to the world's far-flung corners, and the target list expanded — airports, cruise ships, embassies.
Terrorism. The very idea is to strike paralyzing fear into the hearts of an enemy, or a perceived one. To invest an act of violence with far-reaching consequences that go beyond the moment's spectacular display of blood and destruction.
It requires a chilling degree of detachment from the fundamental values upon which civilization is built, ethicists say.
"The world view that undergirds any terrorist activity is as far removed from any ethical philosophy as can be imagined," said Tom Morris, a former Notre Dame University professor of philosophy. "It involves treating people as mere means to ends. It ascribes only instrumental value to people, and no intrinsic value."
The public revulsion inspired by terror attacks is part of the point — but in a way beside the point. Terrorism as we know it today is the product of a media age, a calling card of a global culture in which fame and infamy are closely coupled.
"The whole idea — the only idea, in some sense — is to bring attention to the cause," said Rohan Gunaratna of the Center for the Study of Terrorism and Political Violence in Edinburgh, Scotland.
Sometimes, the battle over terrorism is a struggle over semantics.
"Terrorist" is an epithet, and even the most enthusiastic practitioners of terror's black arts bridle at it. When the State Department periodically updates its list of terror organizations and the countries that sponsor them, this judgment is often accompanies by howls of protest.
But it is a two-sided coin: Repressive regimes use the terrorist tag to discredit those who struggle against them. So who is a terrorist, and who is a freedom fighter?
Even decades after the fact, the answer is sometimes clear, sometimes not. Nelson Mandela is a universally beloved and respected statesman. But with the Mideast's latest plunge into violence, Palestinian leader Yasser Arafat's claim to the title of leader and statesman is being bitterly challenged.
The line between guerrilla warfare and terrorism has always been a difficult one to define.
Confronted with a powerful foe, rebel movements and guerrilla armies use tactics that are in some ways akin to terrorism, relying heavily on stealth, surprise and audacity — and sometimes striking at civilian targets rather than military ones.
"What some people call terrorism would probably be better defined as low-intensity warfare," said Merari, the Tel Aviv University expert.
Tuesday's horrifying strikes against the World Trade Center and the Pentagon — in which fully loaded commercial airliners were wielded as weapons — could herald the start of an era in which terror and retribution are hard to distinguish from all-out war.
Once responsibility has been determined, a commensurate military response is likely, Merari predicted.
"Attacks like these are clearly unprecedented, and I think it will have a tremendous impact on world behavior toward terrorism in general — and these perpetrators in particular," he said.