In
video games specificly
shoot 'em ups of the
platform or
scrolling variety, a weapon that will destroy every enemy on the screen, or at least do a significant amount of damage. You are given a limited supply, and they are very rare as
pickups in most games. Often the only way to get more is to sacrifice a
life.
I'm informed by Ashley Pomeroy that the first game to feature smart bombs was Williams' 'Defender', in 1981.