“The use of the term terrorism delegitimizes the opponent,” said Martha Crenshaw, a scholar at Stanford who wrote her first essay wrestling with the definition of terrorism in 1972. “It’s not just the tactics that are discredited, it’s the cause, as well.”

In fact, accused terrorists often throw the label back at their accusers. In a recording played in court last week, David B. Stone Sr., leader of the Hutaree group, described the government as a “terrorist organization.” And Doku Umarov, the Chechen guerrilla leader who claimed responsibility for the suicide bombings in the Moscow subway, took the same line in a videotaped message, suggesting that the real terrorist was his nemesis, Vladimir V. Putin, the Russian prime minister.

“Any politician or journalist or any person who will condemn me for those operations, or who will accuse me of terrorism, I am laughing at those people,” he said, “because I haven’t heard that Putin was accused of terrorism for the murder of civilians.”

The word originated in the context of large-scale violence by the state: the Jacobin Reign of Terror during the French Revolution, when 16,000 to 40,000 people were killed in 13 months. The Latin root “terrere” means “to cause to tremble,” and one essential notion in most definitions of terrorism is that it seeks to frighten the enemy, as well as to inspire allies.

Over time, terrorism has come to be applied more commonly to the violent tactics of nonstate groups, often in a campaign of repeated attacks. The targets are often chosen for symbolic reasons (the World Trade Center, the Pentagon), and the victims usually include civilians. The acts of terror seek to influence an audience, ostensibly in service of a political goal.

The anarchist movement before and after the turn of the 20th century spoke of the “Propaganda of the Deed,” a phrase that captures both the violence and its purported political purpose. Their deeds included the assassination of numerous politicians and world leaders, including President William McKinley in 1901, and they were the rare militants who did not shun the terrorism label.

“They called themselves terrorists and they were proud of it,” said David C. Rapoport, a historian of terrorism and editor of the journal Terrorism and Political Violence.