belly button vs omphalus

belly button and omphalus both are nouns.

Word	Noun	Adjective	Verb	Adverb
belly button	Yes	No	No	No
omphalus	Yes	No	No	No

As nouns, belly button and omphalus are synonyms defined as:

Other synonyms of belly button include bellybutton, navel, omphalos, umbilicus.

belly button (noun)	omphalus (noun)
a scar where the umbilical cord was attached	a scar where the umbilical cord was attached