Terrorism is speech — speech that gathers its audience by killing innocents as theatrically as possible. The 19th-century anarchist Paul Brousse called it “propaganda by deed.”

Accordingly, the Boston Marathon attack, the first successful terror bombing in the U.S. since 9/11, was designed for maximum effect. At the finish line there would be not only news cameras but also hundreds of personal videos to amplify the message.

But what message? There was no claim of responsibility, no explanatory propaganda. Indeed, was it terrorism at all?

There was much ado about President Obama’s nonuse of the word “terrorism” in his first statement to the nation after the bombing. Indeed, the very next morning, he took to the White House briefing room for no other reason than to pronounce the event an “act of terrorism.”

He justified the update as a response to “what we now know.” But there had been no new information overnight. Nothing changed, except a certain trepidation about the original omission.

There was no need to be so sensitive, however. The president said that terrorism is any bombing aimed at civilians. Not quite. Terrorism is any attack on civilians for a political purpose. Until you know the purpose, you can’t know if it is terrorism.

Sometimes an attack can have no purpose. The Tucson shooter who nearly killed Rep. Gabrielle Giffords was simply deranged, a certified paranoid schizophrenic. Or there might be some personal vendetta — a purpose, but not political. In the Boston case, conceivably a grudge against the marathon, its organizers or something associated with the race.

That, of course, is extremely improbable. (Schizophrenics are too disorganized to set off simultaneous bombs, for example.) It’s overwhelmingly likely that the Boston attack was political, and therefore terrorism.

Nonetheless, the president’s nonuse of the word was no big deal. Why then was he so sensitive that he corrected the omission the next morning?

Answer: Benghazi, in which the administration had been roundly and correctly criticized for refusing to call it terrorism for so long.

There, the word mattered very much. There were two possible explanations for the killing of the four Americans: a deliberate preplanned attack (terrorism), or a spontaneous demonstration gone wild.

The administration tried to peddle the spontaneous demonstration story in order to blame a mob incited by a nutty Coptic American who had made an offensive video. This would have spared the administration any culpability.

To use the word terrorism, meaning deliberate attack, would have undermined the blame-shifting and raised exactly the questions — about warnings ignored, inadequate security, absence of contingency plans — that have dogged the administration for months.

In Boston, the linguistic challenge for the president is quite different. What if this turns out to be the work of Islamists?

Will Obama use the word? His administration obsessively adopts language that extirpates any possible connection between Islam and terrorism.

It insists on calling jihadists “violent extremists” without ever telling us what they’re extreme about. It even classified the Fort Hood shooting, in which the killer screamed “Allahu Akbar” as he murdered 13 people, as “workplace violence.”

In a speech just last month in Jerusalem, the president referred to the rising tide of Egypt’s Muslim Brotherhood and other Islamists as the rise of “non-secular parties.”

Non-secular? Isn’t that a euphemism for “religious,” i.e., Islamist? Yet Obama couldn’t say the word. This is no linguistic triviality. He wouldn’t be tripping over himself to avoid any reference to Islam if it was insignificant.

Obama has performed admirably during the Boston crisis, speaking both reassuringly and with determination. But his wavering over the word terrorism is telling, though in this case unimportant. The real test will come when we learn the motive for the attack.

As of this writing, we don’t know. It is a measure of the emptiness of Obama’s preferred description — “violent extremists” — that, even as we know nothing, it can be applied to the Boston bomber(s).

Which means the designation is meaningless.    

Charles Krauthammer is a syndicated columnist and Fox News commentator. He can be contacted at:

letters@charleskrauthammer.com