Definition of demo disc in US English:

demo disc

(US demo disk)

noun

informal
  • A recording made to demonstrate or display the abilities of a singer, musician, or group, or to promote a new song, style of music, etc., especially to record companies, disc jockeys, and agents; (later also more generally) any disc used for demonstration or promotion of a product, especially a computer program or game.

Origin

1950s; earliest use found in Billboard.