letter bomb vs bomb

letter bomb and bomb both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter bomb	Yes	No	Yes	No
bomb	Yes	No	Yes	No

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

letter bomb: a thin explosive device inside an envelope or package and detonated when opened
bomb: an explosive device fused to explode under specific conditions

As verbs, bomb is a hypernym of letter bomb; that is, bomb is a word with a broader meaning than letter bomb:

Other hypernyms of letter bomb include bombard.

letter bomb (noun)	bomb (noun)
a thin explosive device inside an envelope or package and detonated when opened	an explosive device fused to explode under specific conditions
	strong sealed vessel for measuring heat of combustion
	an event that fails badly or is totally ineffectual

letter bomb (verb)	bomb (verb)
send an explosive to	throw bombs at or attack with bombs
	fail to get a passing grade