duct vs air duct

duct and air duct both are nouns.

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

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

Other hyponyms of duct include air-intake, air passage, airway.

duct (noun)	air duct (noun)
an enclosed conduit for a fluid	a duct that provides ventilation (as in mines)
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