What's the difference between mass murder and terrorism?

So it appears that terrorism and mass murder are sometimes distinguished with an "I know it when I see it" assessment. And when we see the weapon being used, the public often has an easier time deciding who's a terrorist and who's a rogue: Guns are for mass murderers. Bombs -- or "weapons of mass destruction," as the pressure-cooker weapons constructed in Boston are referred to in the charges filed against the suspect -- are the domain of terrorists.

That's the idea that floats around the general population, anyway. As we saw, the U.S. government has clear-cut definitions for both terms, so no problem separating the two, right? Uh, not exactly. Remember terrorism is defined as unlawful force and violence used to "to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives."

Let's take, for example, the 2011 Arizona shooting that injured, among others, former U.S. Rep. Gabrielle Giffords and killed six others. Shooter Jared Lee Loughner was diagnosed, at one point, with schizophrenia, and although there was some discussion of his interest in conspiracy theories, it doesn't appear he was acting on a political or social agenda [source: Follman]. That being said, a targeted shooting of a political figure at a public event certainly could be seen as an attempt to intimidate a government or civilian population.

Examining the Giffords case, ask yourself this: If Loughner had used a homemade bomb -- or even anthrax or ricin -- to target a politician and cause collateral damage, would we (or media outlets, or the government) call him a terrorist?

On the other hand, were the Tsarnaev brothers' acts that much more ideologically motivated than someone like Nidal Malik Hasan, a U.S. Army psychiatrist charged with killing 13 people and wounding more than 30 during a shooting at Fort Hood in 2009? Hasan had reportedly opposed the wars in Iraq and Afghanistan and had attempted to contact al-Qaida [source: Lapidos]. Hasan, however, is slated to be tried in a military court without terrorism charges in 2013.

Internationally, there's also a struggle to define terrorism in relation to mass murder. Anders Behring Breivik, who killed 77 people and injured 242 others in a series of bombings and a mass shootings in Norway in 2011, was charged with committing terrorist acts [source: BBC News]. However, the planning of the attack wasn't regarded as a terrorist action at the time because before 2012, Norway stipulated that more than one person had to be involved for something to be considered a terrorist plot [source: Criscione]. Essentially, Breivik planned a mass murder and then carried out a terrorist act.

So what's the difference between mass murder and terrorism? For now, there seems to be tacit (if not explicit) international agreement that terrorism is spurred by a specific ideology or political motivation, while mass murder is a more indiscriminate act of violence against an innocent population. While there is certainly overlap, we can usually distinguish the two based on the motivation of the suspect.