fission bomb vs nuclear weapon

fission bomb and nuclear weapon both are nouns.

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

As nouns, nuclear weapon is a hypernym of fission bomb; that is, nuclear 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)
nuclear weapon: a weapon of mass destruction whose explosive power derives from a nuclear reaction

Other hypernyms of fission bomb include bomb, atomic weapon.

fission bomb (noun)	nuclear 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