external oblique muscle vs ab

external oblique muscle and ab both are nouns.

Word	Noun	Adjective	Verb	Adverb
external oblique muscle	Yes	No	No	No
ab	Yes	No	No	No

As nouns, ab is a hypernym of external oblique muscle; that is, ab is a word with a broader meaning than external oblique muscle:

external oblique muscle: a diagonally arranged abdominal muscle on either side of the torso
ab: the muscles of the abdomen

Other hypernyms of external oblique muscle include abdominal, abdominal muscle.

external oblique muscle (noun)	ab (noun)
a diagonally arranged abdominal muscle on either side of the torso	the muscles of the abdomen