After playing their regular Swing gigs, musicians like saxophonist Charlie Parker and trumpeter Dizzy Gillespie would meet up for after hour jam sessions to experiment in a new style based on complexity, difficulty, and increased improvisation. This style became known as:

The answer is bebop.

Explanation:

Bebop is known for its use of fast tempos, virtuosic soloing and harmonic experimentation. It was a reaction to swing and other forms of "danceable" jazz; instead, bebop was meant to be simply listened.

Bebop was very influential for future jazz subgenres, such as hard bop, post-bop, jazz-fusion and free jazz.