As security technology expert Bruce Schneier notes here, according to federal statutes, any simple explosive device -- a bomb, grenade, landmine, rocket, etc. -- fits the definition of "weapon of mass destruction."
The statute definition also includes the things most of us think of in this category -- nuclear and biological weapons, radiological weapons, etc. But it includes everything else too.
That said, to date, in conventional warfare and terrorism, it's old fashioned explosives that kill the overwhelming number of people. The appearances of actual WMD (chem, bio, nuclear, etc.) anywhere is extremely rare. And even then it's seldom proved more lethal than the most up-to-date forms of conventional explosives.