megaton bomb vs nuclear weapon

megaton bomb and nuclear weapon both are nouns.

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

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

megaton bomb: a nuclear weapon with an explosive power equivalent to one million tons of TNT
nuclear weapon: a weapon of mass destruction whose explosive power derives from a nuclear reaction

Other hypernyms of megaton bomb include bomb, atomic weapon.

megaton bomb (noun)	nuclear weapon (noun)
a nuclear weapon with an explosive power equivalent to one million tons of TNT	a weapon of mass destruction whose explosive power derives from a nuclear reaction