abdominal muscle vs ab

abdominal muscle and ab both are nouns.

Word	Noun	Adjective	Verb	Adverb
abdominal muscle	Yes	No	No	No
ab	Yes	No	No	No

As nouns, abdominal muscle and ab are synonyms defined as:

Other synonyms of abdominal muscle include abdominal.

abdominal muscle (noun)	ab (noun)
the muscles of the abdomen	the muscles of the abdomen