Many press accounts of the Boston Marathon bombings seized on the moment when President Obama used the word "terror" to describe the events. Some thought it was about time, while others speculated whether this move was a simple political calculation. Senator McCaskill, though, was interested in what exactly the difference between the Boston attacks and the Sandy Hook shooting was — in other words, what made the Boston bombing terrorism, while Sandy Hook was not? This is an interesting question, and to answer it we should consider several elements: what do we know about the two events, what is the political context, and what exactly it was that Mr. Obama said. Before we look at those elements, though, let's establish a definition of terrorism, to the extent possible.
The most common element in the definition of terrorism is the use or threat of violence for political ends. This misses something, though — this definition would include all acts in a war, for example. While some may view all war as a form of terrorism, mainstream usage indicates that terrorism is different. For the purposes of discussion, then, I'll add the element of deliberately and explicitly targeting innocent civilians. I add explicitly because it differentiates events where civilians may have been killed by accident (for example, an airstrike targeting militants) from events such as 9/11 where there was not even a pretense of a military target.
The core of Sen. McCaskill's critique seems to be that President Obama (and Secretary of Defense Chuck Hagel) was willing to call the attacks in Boston terrorism, without knowing that the motivation was in fact different from the attack at Sandy Hook, which was not called terrorism. Since we don't yet know who is responsible (though we might soon), we cannot know the motivation — political or otherwise. If we didn't call Sandy Hook terrorism because there seemed to be no political aim, how can we call the Boston attacks terrorism without knowing the same? All we do know is that someone targeted innocent civilians with violence. This certainly induces a feeling of terror, but absent the political aims, it does not quite match out usual definition of terrorism (Napolitano seems to admit this much in her response).
The question of political context is relevant because comments by government officials regarding high-profile events are inevitably subject to political interpretation, and statements are thus made with this in mind. In this particular context, the fact that the Obama Administration seemed reluctant to refer to the Benghazi attack as "terrorism" and was criticized in the media as a result is particularly prominent. Indeed, a number of pundits seemed quite eager to find out whether Obama would use this word.
So it appears that a case could be made that the word itself is being watered down. Sure, the attack in Boston deliberately, explicitly targeted civilians by implication — it was a bomb in a crowded area, after all. But we don't know who did it, and thus we can't know the motivation, political or otherwise. At this point, though, it's worth pointing out that what President Obama actually said was that the attacks were being "investigated as" an act of terror, presumably describing their investigative techniques rather than the act itself. Is this too nit-picky to save "terrorism" from being watered down?
I would argue that, if it emerges that there is clearly no political motive — that this incident is more like Sandy Hook than Oklahoma City — the administration should acknowledge this and revise their assessment. Just as denying a clear act of terrorism does us no favors, overusing the term to the point of meaninglessness makes productive discussion more difficult. For now, hedging by saying that the incident is being "investigated as" an act of terror is fair, since the attack seems to have all the traditional hallmarks of terrorism. (Indeed, it seems rare for a public bombing of a crowded civilian area to occur that does not end up having been a clear case of terrorism.) To do otherwise at this point is to do a disservice to our language, and our discourse.