crew neck vs crew neckline

crew neck and crew neckline both are nouns.

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

As nouns, crew neck and crew neckline are synonyms defined as:

crew neck and crew neckline: a plain straight neckline opening from shoulder to shoulder of sweaters

crew neck (noun)	crew neckline (noun)
a plain straight neckline opening from shoulder to shoulder of sweaters	a plain straight neckline opening from shoulder to shoulder of sweaters