Memory device and Optical disc

Memory device noun - A device that preserves information for retrieval.

Optical disc and memory device are semantically related. In some cases you can replace phrase "Optical disc" with "Memory device", this phrases are similar.

Optical disc 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.

Memory device and optical disc are semantically related. You can use "Memory device" instead a noun phrase "Optical disc".