fission bomb vs atomic weapon

fission bomb and atomic weapon both are nouns.

Word	Noun	Adjective	Verb	Adverb
fission bomb	Yes	No	No	No
atomic weapon	Yes	No	No	No

As nouns, atomic weapon is a hypernym of fission bomb; that is, atomic weapon is a word with a broader meaning than fission bomb:

fission bomb: a nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element like uranium 235 or plutonium 239)
atomic weapon: a weapon of mass destruction whose explosive power derives from a nuclear reaction

Other hypernyms of fission bomb include bomb, nuclear weapon.

fission bomb (noun)	atomic weapon (noun)
a nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element like uranium 235 or plutonium 239)	a weapon of mass destruction whose explosive power derives from a nuclear reaction