pectus vs female chest

pectus and female chest both are nouns.

Word	Noun	Adjective	Verb	Adverb
pectus	Yes	No	No	No
female chest	Yes	No	No	No

As nouns, female chest is a hyponym of pectus; that is, female chest is a word with a more specific, narrower meaning than pectus:

pectus: the part of the human torso between the neck and the diaphragm or the corresponding part in other vertebrates
female chest: the chest of a woman

Other hyponyms of pectus include bust, male chest.

pectus (noun)	female chest (noun)
the part of the human torso between the neck and the diaphragm or the corresponding part in other vertebrates	the chest of a woman