ab vs abdominal muscle

ab and abdominal muscle both are nouns.

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

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

Other synonyms of ab include abdominal.

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