musculus pectoralis vs pecs

musculus pectoralis and pecs both are nouns.

Word	Noun	Adjective	Verb	Adverb
musculus pectoralis	Yes	No	No	No
pecs	Yes	No	No	No

As nouns, musculus pectoralis and pecs are synonyms defined as:

Other synonyms of musculus pectoralis include pectoral, pectoral muscle, pectoralis.

musculus pectoralis (noun)	pecs (noun)
either of two large muscles of the chest	either of two large muscles of the chest