Memory device and Optical disk

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

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

Optical disk 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 disk are semantically related. Sometimes you can use "Memory device" instead a noun phrase "Optical disk".