The term covers subgenres like hard bop and free jazz.
"Post Bop is a modern jazz style that continues the distinguishing characteristics that separate jazz from the world of pop and rock; swing rhythm and extended harmonies (9th chords 11ths, altered chords, etc). Post Bop grew out of the Hard Bop genre during the early to mid-60s as musicians such as Bill Evans, Wayne Shorter, and Herbie Hancock began to introduce more extended harmonies, abstract structures, and looser rhythms in their playing and compositions."
From Essential Tools for Post-Bop Cutting Edge Jazz Improvisation by Olegario Diaz
"Forms, tempos, and meters are freer, all the compositions are new, and the band members themselves are featured composers.... [A]n approach that is abstract and intense in the extreme, with space created for rhythmic and coloristic independence of the drummer—an approach that incorporated modal and chordal harmonies, flexible form, structured choruses, melodic variation, and free improvisation."
Musicologist Jeremy Yudkin
In 1969, Miles Davis's producer Teo Marcero sent a memo to executives at Columbia Records which read:
"Miles just called and said he wants this album to be titled: Bitches Brew.
Keyboardist, bandleader, and jazz innovator.
A jazz maverick and creative force in music 40+ years.
His motto was "There are no wrong notes on the piano."
The sound of young America masterminded by Berry Gordy.
Combines rhythm and blues and gospel music styles.
Soul gets high.
Rock loses the roll but gains amplitude and attitude.