optical disk vs DVD

optical disk and DVD both are nouns.

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

As nouns, DVD is a hyponym of optical disk; that is, DVD 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
DVD: a digital recording (as of a movie) on an optical disk that can be played on a computer or a television set

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

optical disk (noun)	DVD (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 digital recording (as of a movie) on an optical disk that can be played on a computer or a television set