optical disk vs memory device

optical disk and memory device both are nouns.

Word	Noun	Adjective	Verb	Adverb
optical disk	Yes	No	No	No
memory device	Yes	No	No	No

As nouns, memory device is a hypernym of optical disk; that is, memory device is a word with a broader 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
memory device: a device that preserves information for retrieval

Other hypernyms of optical disk include storage device.

optical disk (noun)	memory device (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 device that preserves information for retrieval