neckline vs neck opening

neckline and neck opening both are nouns.

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

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

neckline: the line formed by the edge of a garment around the neck
neck opening: an opening in a garment for the neck of the wearer; a part of the garment near the wearer's neck

Other hypernyms of neckline include neck.

neckline (noun)	neck opening (noun)
the line formed by the edge of a garment around the neck	an opening in a garment for the neck of the wearer; a part of the garment near the wearer's neck