Third stream is a term coined in 1957 by composer Gunther Schuller to describe a musical genre which is a synthesis of classical music and jazz. Improvisation — a key element of jazz, but far less common in classical music — is generally seen as a vital component of Third Stream.

In 1961, Schuller defined Third Stream as "a new genre of music located about halfway between jazz and classical music." (Schuller, 114) Schuller insisted that "by definition there is no such thing as 'Third Stream Jazz.'" (Schuller, 120)

Schuller noted that while purists on both sides of Third Stream objected to tainting their favorite music with the other, more strenuous objections were typically made by jazz musicians who felt such efforts were "an assault on their traditions." Schuller writes that "by designating the music as a 'separate, Third Stream', the other two mainstreams could go about their way unaffected by the attempts at fusion." (Schuller, 115) Because Third Stream draws on classical as much as jazz, it is generally required that composers and performers be proficient in both genres.

Critics have argued that Third Stream—by drawing on two very different styles—dilutes the power of each in combining them. (Schuller, 1986) Others reject such notions, and consider Third Stream an interesting musical development. (Schuller, 1986)

In 1981, Schuller offered a list of "What 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. (Schuller, 120)