Here is a look at some of the issues that make the term terrorism so nebulous and tricky.

How did the term originate?

It is derived from the Latin “terrorem,” which translates as “panic,” “alarm” and “great fear.” The term’s use widened with the 1793-94 period of the French Revolution known as the “Reign of Terror,” when the revolutionary government conducted mass executions to intimidate suspected opponents.

How does the dictionary define terrorism?

The Oxford Dictionary calls terrorism “the unlawful use of violence and intimidation, especially against civilians, in the pursuit of political aims.” Merriam-Webster calls it the systematic and coercive use of terror, defined as “violent or destructive acts (such as bombing) committed by groups in order to intimidate a population or government into granting their demands.”

Such definitions are not universally accepted and can be interpreted in different ways. Under the dictionary criteria, some experts would argue, the revolutionary colonist soldiers who slaughtered Native Americans allied with the British were terrorists, as were John Brown’s abolitionists who indiscriminately killed civilians in pro-slavery states before the Civil War. Palestinians regarded as terrorists by Israel are seen by their supporters as resistance fighters opposed to an occupation.

Still, some scholars say the word terrorism can still be used objectively.

“I think it can be defined with a fair degree of precision, which doesn’t necessarily entail judging or taking a position,” said Todd Gitlin, a professor at the Columbia Journalism School. “My definition of terrorism, which I don’t think is unusual, is a violent act in order to strike terror in the hearts of a population toward a political end.”

Has terrorism taken on a new meaning?

Acts of violence have become so ubiquitous that sometimes the terrorism label is applied — as in the Belgium case — even without an obvious ideological purpose. But sometimes the label is not applied — as in the Canada example — and that is where the confusion comes in.