The Collaborative International Dictionary of English v.0.48:

H-bomb \H"-bomb`\ ([=a]ch"b[o^]mb`) n.
   The hydrogen bomb, a thermonuclear weapon that releases
   atomic energy by union of hydrogen nuclei at high
   temperatures to form helium. The force of its explosion may
   range from one to hundreds of megatons of TNT equivalent.

   Syn: hydrogen bomb, fusion bomb, thermonuclear bomb.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

H-bomb
    n 1: a nuclear weapon that releases atomic energy by union of
         light (hydrogen) nuclei at high temperatures to form helium
         [syn: hydrogen bomb, H-bomb, fusion bomb,
         thermonuclear bomb]