surname vs maiden name

surname and maiden name both are nouns.

Word	Noun	Adjective	Verb	Adverb
surname	Yes	No	No	No
maiden name	Yes	No	No	No

As nouns, maiden name is a hyponym of surname; that is, maiden name is a word with a more specific, narrower meaning than surname:

surname: the name used to identify the members of a family (as distinguished from each member's given name)
maiden name: a woman's surname before marriage

surname (noun)	maiden name (noun)
the name used to identify the members of a family (as distinguished from each member's given name)	a woman's surname before marriage