neckline vs decolletage

neckline and decolletage both are nouns.

Word	Noun	Adjective	Verb	Adverb
neckline	Yes	No	No	No
decolletage	Yes	No	No	No

As nouns, decolletage is a hyponym of neckline; that is, decolletage is a word with a more specific, narrower meaning than neckline:

Other hyponyms of neckline include crew neck, crew neckline, keyhole neck, keyhole neckline.

neckline (noun)	decolletage (noun)
the line formed by the edge of a garment around the neck	a low-cut neckline on a woman's dress