Dzhokhar Tsarnaev, the surviving suspect in the Boston Marathon bombing, was charged today with using a weapon of mass destruction. It's yet another circumstance where the legal and colloquial definitions of "weapon of mass destruction" are at odds.

The actual bomb Tsarnaev allegedly constructed and detonated is pretty much the opposite of what people think about when they think "weapon of mass destruction," a vague term that usually means a weapon carrying an unconventional payload, like a nuclear, chemical or biological yield. The FBI affiant, Special Agent Daniel Genck, confirms the bombs used pressure cookers for their hulls – "of the same brand" – packed with "low grade explosive" containing BBs and nails and a "green hobby fuse."

Bashar Assad's chemical arsenal this ain't. But, as Danger Room explained after U.S. citizen and anti-Assad fighter Eric Harroun, faced similar charges, "weapon of mass destruction" is a very broad category under federal law. Grenades, mines, missiles and rockets all apply. So do homemade bombs of the sort Tsarnaev allegedly constructed. About all that doesn't apply are firearms and pyrotechnics gear. No one ever said the law had to coincide with military terminology.

We've argued all this helps speak to the definitional absurdity surrounding "weapons of mass destruction," and indicates the infamous term ought to be retired, replaced instead by the specifics of what an explosive actually is or does. None of that bears on Tsarnaev's case.

But Genck's affidavit shows that the type of bomb that killed three and wounded approximately 180 at the Boston Marathon April 15 also helped build the government's case against Tsarnaev. The explosive devices used at the scene of Friday morning's wild Watertown battle between the brothers and police aligned with the composition of those used at the Marathon, down to the "green-colored hobby fuse." And while the accused bomber was in the hospital yesterday, the FBI searched his University of Massachusetts-Dartmouth dormitory, the affidavit states, and found "a large pyrotechnic, a black jacket and a white hat" similar to the kind Suspect Two wore on the surveillance footage of the Marathon that agents watched.

Tsarnaev is innocent until proven guilty in a court of law. There appears to be no similar redress scheduled for the way in which the law contorts a common-sense meaning of an already vague term about some of the world's deadliest weapons.