The term '40s Jazz Style' refers to two distinct jazz styles that emerged in the 1940s: BOP and BEBOP. BOP, also known as 'Rebop, ' is a style of jazz characterized by fast tempos, improvisation, and complex harmonic structures. BEBOP is a more modern style of jazz that developed from BOP and is characterized by its use of complex chord progressions and improvisation. Both styles are still popular today and have had a major influence on the development of jazz music.