compact disc vs compact disk

compact disc

noun
  • A kind of optical disc used to store audio or other data. 

compact disk

noun
  • A form of digital media on a plastic disc read by a laser in a CD drive. It comes in several varieties including CD-ROM, CD-R, and CD-RW. 

How often have the words compact disc and compact disk occurred in a corpus of books? (source: Google Ngram Viewer )