Susan Walsh/AP Photo

The word "terrorism" is now officially attached to the Boston Marathon bombing, after President Obama used the word Tuesday morning in his address from the White House.

"This was a heinous and cowardly act. And given what we now know about what took place, the FBI is investigating it as an act of terrorism," Obama said. "Any time bombs are used to target innocent civilians it is an act of terror. What we don't yet know, however, is who carried out this attack, or why."

It was the first time Obama had used that language about the boming.

On Monday, when he addressed the nation for the first time after two bombs exploded in Boston, Obama spoke in measured, deliberate tones about his conversations with Massachusetts Gov. Deval Patrick and Boston Mayor Tom Menino, and he sternly vowed justice for whoever was responsible, but he didn't use the word "terrorism."

Soon after, a White House official told ABC News that any bombing of this kind would be considered an act of terrorism. Senators on the Homeland Security Committee said the attacks bore the hallmarks of terrorism, and Intelligence Committee Chairwoman Dianne Feinstein, D-Calif., told ABC News: "It is a terrorist incident."

Following the president's lead, Rep. Michael Capuano, D-Mass., shied away from the "T" word, saying on the House floor before a moment of silence that "Whatever it was, it was a terrible tragedy … no matter how you measure it."

Regardless of why Obama waited a day to call the attacks "terrorism," what makes something a terrorist event?

According to David Gomez, a former FBI counterterrorism official, Justice Department protocol automatically classifies bombings of this nature as terrorism cases, partly for jurisdictional purposes: Until deemed otherwise, the FBI (and not Bureau of Alcohol, Tobacco, Firearms and Explosives) handles bombings as terrorism investigations.

"There's a number of different definitions for 'terrorism.' There's the academic definition, and there are the legal definitions, and then there's the one that's used by the FBI, which kind of straddles both," Gomez told ABC News. "The FBI uses a definition that talks about coercing the civilian population or the government … through violence. … It's an attempt to change their minds about certain things."

A former senior assistant special agent-in-charge for counter-terrorism and intelligence at the FBI's Seattle Field Office, Gomez said the word "terrorism" carries political connotations about a perpetrator's motive. Newtown, Conn., shooter Adam Lanza, for instance, was not a terrorist because he wasn't trying to communicate a message to the public or the government.

"He was sending a message, but it wasn't about politics. It was about his own mental illness," Gomez said.

Under Gomez's definition, the Boston attacks will be aptly deemed "terrorism" if the assumed motives of public intimidation are found to be correct.

"For example, what if the guy who [placed] the bomb was trying to murder a specific person in the race?" Gomez asked. "That's not terrorism - he was not trying to send a message or influence policy."

Forensic investigators don't think about the word, Gomez told ABC.

"The guys at the crime scene don't care what label you put on it," Gomez said. "They're concerned with solving a specific crime, and that's bombing and murder."