crew neck vs neckline

crew neck and neckline both are nouns.

Word	Noun	Adjective	Verb	Adverb
crew neck	Yes	No	No	No
neckline	Yes	No	No	No

As nouns, neckline is a hypernym of crew neck; that is, neckline is a word with a broader meaning than crew neck:

crew neck: a plain straight neckline opening from shoulder to shoulder of sweaters
neckline: the line formed by the edge of a garment around the neck

crew neck (noun)	neckline (noun)
a plain straight neckline opening from shoulder to shoulder of sweaters	the line formed by the edge of a garment around the neck