Third Stream is a term coined in 1957 … to describe a musical genre that is a synthesis of classical music and jazz.

…Third Stream is not:
– It is not jazz with strings.
– It is not jazz played on ‘classical’ instruments.
– It is not classical music played by jazz players.
– It is not inserting a bit of Ravel or Schoenberg between be-bop changes—nor the reverse.
– It is not jazz in fugal form.
– It is not a fugue played by jazz players.
– It is not designed to do away with jazz or classical music; it is just another option amongst many for today’s creative musicians.

— Wikipedia