sobriquet vs cognomen

sobriquet and cognomen both are nouns.

Word	Noun	Adjective	Verb	Adverb
sobriquet	Yes	No	No	No
cognomen	Yes	No	No	No

As nouns, sobriquet and cognomen are synonyms defined as:

sobriquet and cognomen: a familiar name for a person (often a shortened version of a person's given name)

Other synonyms of sobriquet include byname, moniker, nickname, soubriquet.

sobriquet (noun)	cognomen (noun)
a familiar name for a person (often a shortened version of a person'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)