last name vs cognomen

last name and cognomen both are nouns.

Word	Noun	Adjective	Verb	Adverb
last name	Yes	No	No	No
cognomen	Yes	No	No	No

As nouns, last name and cognomen are synonyms defined as:

last name and cognomen: the name used to identify the members of a family (as distinguished from each member's given name)

Other synonyms of last name include family name, surname.

last name (noun)	cognomen (noun)
the name used to identify the members of a family (as distinguished from each member's given name)	the name used to identify the members of a family (as distinguished from each member's given name)
	a familiar name for a person (often a shortened version of a person's given name)