A-bomb vs nuclear weapon

A-bomb and nuclear weapon both are nouns.

Word	Noun	Adjective	Verb	Adverb
A-bomb	Yes	No	No	No
nuclear weapon	Yes	No	No	No

As nouns, nuclear weapon is a hypernym of A-bomb; that is, nuclear weapon is a word with a broader meaning than A-bomb:

A-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 A-bomb include bomb, atomic weapon.

A-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