Cool jazz is a style of modern jazz music inspired by bebop and big band that arose in the United States after World War II. It is characterized by relaxed tempos and a lighter tone than that used in the fast and complex bebop style. Cool jazz often employs formal arrangements and incorporates elements of classical music. Broadly, the genre refers to a number of post-war jazz styles employing a more subdued approach than that of contemporaneous jazz idioms. As Paul Tanner, Maurice Gerow, and David Megill suggest, "the tonal sonorities of these conservative players could be compared to pastel colors, while the solos of [Dizzy] Gillespie and his followers could be compared to fiery red colors."

The term cool started being applied to this music around 1953, when Capitol Records released the album Classics in Jazz: Cool and Quiet. Mark C. Gridley, writing in the All Music Guide to Jazz, identifies four overlapping sub-categories of cool jazz:

"Soft variants of bebop," including the Miles Davis recordings that constitute Birth of the Cool; the complete works of the Modern Jazz Quartet; the output of Gerry Mulligan, especially his work with Chet Baker and Bob Brookmeyer; the music of Stan Kenton's sidemen during the late 1940s through the 1950s; and the works of George Shearing and Stan Getz. The output of modern players who eschewed bebop in favor of advanced swing-era developments, including Lennie Tristano, Lee Konitz, and Warne Marsh; Dave Brubeck and Paul Desmond; and performers such as Jimmy Giuffre and Dave Pell who were influenced by Count Basie and Lester Young's small-group music. Musicians from either of the previous categories who were active in California from the 1940s through the 1960s, developing what came to be known as West Coast jazz. "Exploratory music with a subdued effect by Teddy Charles, Chico Hamilton, John LaPorta, and their colleagues during the 1950s."