adj

• (music) Describing Western music and musicians of the late 18th and early 19th centuries.
• (informal, music) Describing art music (rather than pop, jazz, blues, etc), especially when played using instruments of the orchestra.
• (physics) Pertaining to models of physical laws that do not take quantum or relativistic effects into account; Newtonian or Maxwellian.

noun

• (countable) One that is classical in some way; for example, a classical economist.
  "Similarly, the new classicals never claimed to be Austrians, nor did they ever make the attempt to meet Austrian objections. Therefore, we cannot fault them for not using this methodology. Nevertheless, new classicals constantly preach […]"
• (chess) Ellipsis of classical chess.
  "When I want to have fun I play blitz. When I want to go deeper and work on improving my game, I play classical and I try to do it in tournament settings."