pectoralis vs skeletal muscle

pectoralis and skeletal muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
pectoralis	Yes	No	No	No
skeletal muscle	Yes	No	No	No

As nouns, skeletal muscle is a hypernym of pectoralis; that is, skeletal muscle is a word with a broader meaning than pectoralis:

pectoralis: either of two large muscles of the chest
skeletal muscle: a muscle that is connected at either or both ends to a bone and so move parts of the skeleton; a muscle that is characterized by transverse stripes

Other hypernyms of pectoralis include striated muscle.

pectoralis (noun)	skeletal muscle (noun)
either of two large muscles of the chest	a muscle that is connected at either or both ends to a bone and so move parts of the skeleton; a muscle that is characterized by transverse stripes