optical disk vs CD

optical disk and CD both are nouns.

Word	Noun	Adjective	Verb	Adverb
optical disk	Yes	No	No	No
CD	Yes	No	No	No

As nouns, CD is a hyponym of optical disk; that is, CD is a word with a more specific, narrower meaning than optical disk:

optical disk: a disk coated with plastic that can store digital data as tiny pits etched in the surface; is read with a laser that scans the surface
CD: a digitally encoded recording on an optical disk that is smaller than a phonograph record; played back by a laser

Other hyponyms of optical disk include compact disc, compact disk, DVD, videodisc, videodisk.

optical disk (noun)	CD (noun)
a disk coated with plastic that can store digital data as tiny pits etched in the surface; is read with a laser that scans the surface	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