The term '40s Jazz' refers to a style of jazz music that emerged in the 1940s. This style is characterized by improvisation, complex harmonic structures, and a fast tempo. The term 'BOP' is often used to refer to this style of jazz, as it was popularized by BEBOP musicians such as Charlie Parker and Dizzy Gillespie. BOP is an acronym for 'BEBOP', which is a style of jazz characterized by fast tempos, improvisation, and complex harmonic structures.