The Boston Marathon bombing once again has Americans asking, "Is this terrorism?" And more broadly, "What is terrorism?"

We asked ourselves those questions the moment we heard the news. Our news anchors asked for the next 24 hours (though they were clear to say at first that they did not know if it was terrorism). President Obama - thankfully - was careful not to use the word in his first press conference on the day of the bombings. But later on the following day - on Tuesday - Obama said, "Any time bombs are used to target civilians, it is an act of terrorism."

What does it mean when we say something is "terrorism" and why does it matter?

As a professor of Sociology and Law, I study how ordinary people understand the law and how the law itself, including law's categories and terms affect how people understand the world around them.

Using terms like "terrorism" shapes what ordinary people expect of the police, the justice system and our government. It affects what kind of punishment we want and the level of fear we feel about what is going on around us.

But what is terrorism? The most obvious definition is that terrorism is a crime meant to terrorise. We know what these kinds of crimes are: they are the kind that make us afraid to send our children to school, like Columbine; make us afraid to go to work, like 9/11; or make us afraid even just to spend beautiful spring day competing with our friends competing in a footrace.

Components of terrorism

And yet, we do not tend to think of Columbine as a terrorist act. It was a couple of kids with serious mental health problems and illegal access to guns and ammunition. Similarly, when some unknown person or persons poisoned a few bottles of Tylenol in Chicago in 1982, the US was terrorised. We do not tend to think of that incident as "terrorism", however. Why? Perhaps it is because we do not know the motivation of the criminal(s) since they have never been caught. Or, it could be because, despite the nation-wide response, the crime was most likely committed by an individual placing the poison in bottles already on store shelves.

Terrorising, therefore, is clearly not the only component of terrorism.

Maybe terrorism requires crimes committed for political reasons. The FBI admits there is no "standard, accepted definition of terrorism", but this is what constitutes terrorism for the FBI because US law defines terrorism as, "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" (28 CFR Section 0.85).

So many crimes meet this definition but that we would not call "terrorism". When a woman kills her husband because the police don't believe he is capable of domestic violence and won't protect her, it is a personal act of safety and a political act designed to change society.

Ex-CIA officer discusses Boston

Marathon bombings

When civil rights activists led by Rev Martin Luther King, Jr, illegally marched in the streets or sat at lunch counters marked "whites only", to help topple Jim Crow and transition the US into the modern era, they were most certainly using force for a political agenda. Called terrorists at the time, civil rights activists disavowed violence, but certainly used force and violence erupted. ACT UP famously brought HIV/AIDS to public and political notice by assaulting politicians by spilling real and fake blood on them in public places. While disruptive and unsettling at the time, was this terrorism? They were called terrorists at the time.

Terrorising plus force plus political or social motives does not always constitute terrorism, therefore.

Is terrorism defined by criminal action plus violence plus affiliation with some sort of group? Violent al-Qaeda actions clearly qualify as terrorism, but what about the Atlanta Olympic bomber Eric Rudolph? He was part of a political and social movement by his own admission. He was dedicated to illustrating the godlessness of the US.

President Clinton labelled the bombing "an evil act of terror" the day after the bombing, although it took months to determine that Rudolph did it and even longer to find him. His movement rejected him, and while the Olympics and the US more generally were "terrorised", he was not convicted of "terrorism". And yet, the Atlanta bombing always comes up as an example of domestic terrorism, Rudolph was an extremist rejected by his own extremist religious organisations. He had political motives, but they largely existed in his mind alone.

Using the word 'terrorist'

Similarly, Timothy McVeigh and his co-conspirators (a total of four) were part of the "militia movement" but there was no real organised group supporting that destructive crime. They had beliefs that originated in and resonated with particular movements, but they did not represent that movement.

On Tuesday, President Obama offered that, "anytime bombs are used to target innocent civilians, it is an act of terrorism" which provides another possibility: that terrorism is related to the type of force used. Public bombings, then, would always constitute terrorism. But how does this help us understand what is going on here? And why is a knife any different than a bomb if the effect is similar?

One reason to use the word "terrorist" is simply pragmatic. When the police say this incident will be "investigated and treated like terrorism", it could indicate that more sophisticated resources will be brought to bear to find the perpetrator(s). A major industry certainly has developed around terrorism experts, responders and planners which may or may not be keeping us safer.

What work is the word "terrorism" doing in these conversations? Is it helping us to make sense of things or is it subtly becoming a kind of shorthand for the people we do not like and the motives that are unfavourable? Terrorism has come to signify race and religion though everyone is careful not to say so.

I am not the first person to question the definition and use of this word, but, like most Americans, I want to know who did this and why. And I want them brought to justice because on Tuesday, in Boston, a bomber or bombers committed multiple heinous murders and attempted murders. He, she or they endangered lives in public, committed assault, possession of explosives, and a host of crimes that District Attorneys are itching to charge. And right now, the use of the word "terrorism" is not helping us get there.

Laura Beth Nielsen is Associate Professor of Sociology and Director of Legal Studies, Northwestern University, and research professor for the American Bar Foundation. She is the author of License to Harass: Law, Hierarchy and Offensive Public Speech and is part of The OpEd Project's Public Voices Fellowship at NU.

Follow her on Twitter: @ProfLBNielsen