penis vs erection

penis and erection both are nouns.

Word	Noun	Adjective	Verb	Adverb
penis	Yes	No	No	No
erection	Yes	No	No	No

As nouns, erection is a hyponym of penis; that is, erection is a word with a more specific, narrower meaning than penis:

Other hyponyms of penis include cock, dick, pecker, prick, putz, shaft, tool, micropenis, microphallus, hard-on.

penis (noun)	erection (noun)
the male sex organ	the act of building or putting up
	a structure that has been erected
	an erect penis