air duct vs duct

air duct and duct both are nouns.

Word	Noun	Adjective	Verb	Adverb
air duct	Yes	No	No	No
duct	Yes	No	No	No

As nouns, duct is a hypernym of air duct; that is, duct is a word with a broader meaning than air duct:

air duct (noun)	duct (noun)
a duct that provides ventilation (as in mines)	an enclosed conduit for a fluid
	a bodily passage or tube lined with epithelial cells and conveying a secretion or other substance
	a continuous tube formed by a row of elongated cells lacking intervening end walls