audio CD vs CD

audio CD and CD both are nouns.

Word	Noun	Adjective	Verb	Adverb
audio CD	Yes	No	No	No
CD	Yes	No	No	No

As nouns, CD is a hypernym of audio CD; that is, CD is a word with a broader meaning than audio CD:

audio CD: compact discs used to reproduce sound (voice and music)
CD: a digitally encoded recording on an optical disk that is smaller than a phonograph record; played back by a laser

Other hypernyms of audio CD include compact disc, compact disk.

audio CD (noun)	CD (noun)
compact discs used to reproduce sound (voice and music)	a digitally encoded recording on an optical disk that is smaller than a phonograph record; played back by a laser
	a debt instrument issued by a bank; usually pays interest