That includes the U.S. government. "No one definition of terrorism has gained universal acceptance," the State Department said in a report on world terrorism in 2000.

The key elements to terrorism are obvious to many violence, noncombatant targets, intention of spreading fear, and political aims. But crafting a watertight, commonly accepted definition has proven difficult.

Are attacks on military personnel or political leaders acts of terrorism? What about attacks on property? Must terrorism be physically violent, or is it enough to simply instill fear?

FBI, State Department Look to Different Definitions

Even among U.S. governmental agencies, different definitions of terrorism are used.

The State Department's definition holds that only sub-national groups, not states themselves, can commit acts of terrorism. It states the violence must be politically motivated, but does not mention instilling or spreading fear.

The Federal Emergency Management Agency's definition includes the use of illegal force or violence "for purposes of intimidation, coercion or ransom," but does not require it to be politically motivated.

The FBI looks to the Code of Federal Regulations definition: "The unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives."

The FBI has labeled as terrorists groups such as the Earth Liberation Front, which has taken responsibility for destroying millions of dollars worth of property, but claims to be nonviolent and avoid hurting people.

After the Sept. 11 attacks on New York and Washington, there is more pressure to agree on one particular definition.

In the Eye of the Beholder?

"There has never been any consensus definition of terrorism," says Richard Betts, director of the Institute of War and Peace Studies at Columbia University.

The principal problem, Betts says, is that many people believe there are cases where extreme actions are justified. But these people also believe the terrorism is always wrong.

Betts notes, for example, that the United States bombed civilian targets in World War II, but few would call it terrorism. The action is widely considered justified by the fact that America had been drawn into a war with aggressor nations.

"Everybody can find an exception to any abstract definition," he says.

Some critics have suggested the word is too often simply applied to whatever violent groups the United States opposes, pointing to the often-cited claim that "One man's terrorist is another man's freedom fighter."

Michael Kinsley voiced a related skepticism in a recent column in his online magazine Slate. "The most accurate definition of terrorism may be the famous Potter Stewart standard of obscenity: 'I know it when I see it,'" Kinsley wrote.

Many See Terrorism as Clear-Cut

Others are more certain that terrorism can be sharply and fairly defined, however.

"In a nutshell, [terrorism] is the threat and use of both psychological and physical force in violation of international law, by state and sub-state agencies for strategic and political goals," says Yonah Alexander, a terrorism expert and director of the Institute for Studies in International Terrorism at the State University of New York.

"No ifs, ands, or buts," he adds.

Alexander sees international law as the key to separating legitimate use of force from terrorism. If insurgents are fighting a "lawful war" using tactics accepted by international law, they are not terrorists.

"Terrorists are not insurgents, not guerrillas," he stresses. "Terrorists are beyond all norms. They don't recognize any laws."

This is the essential difference between terrorism and other violence, he says, and why there is no merit to claims by the Taliban, Osama bin Laden and his al Qaeda network that the U.S. bombing of Afghanistan is a terrorist act itself.

Michael Barnett, a political science professor at the University of Wisconsin, also takes a hard line on defining terrorism, saying the choice of targets is key in deciding if something was a terrorist act.

"For me, the critical issue is this question of whether the immediate target is civilian," he says.

Barnett admits his definition leaves room for the question of whether the United States has ever in its history engaged in terrorist acts itself.

Donna Jo Napoli, a Swarthmore College linguistics professor, looks to the word's roots for what she sees a clear and unambiguous meaning.

"When you say terrorism you do mean 'trafficking in terror,'" she says. "Anything that could terrorize people."