Big Band refers to a jazz group of ten or more musicians, usually featuring at least three trumpets, two or more trombones, four or more saxophones, and a "rhythm section" of accompanists playing some combination of piano, guitar, bass, and drums. "Big band music" as a concept for music fans is identified most with the swing era, although there were large, jazz-oriented dance bands before the swing era of the 1930s and '40s, and large jazz-oriented concert bands after the swing era.
Classification difficulties occur when stores shelve recordings by all large jazz ensembles as though it were a single style, despite the shifting harmonic and rhythmic approaches employed by new ensembles of similar instrumentation that have formed since the swing era. By lumping the music of all large jazz bands together, marketers overlook the different kinds of jazz that large groups have performed: swing (Duke Ellington and Count Basie), bebop (Dizzy Gillespie), cool (Gerry Mulligan, Shorty Rogers, Gil Evans), hard bop (Gerald Wilson), free jazz (some of Sun Ra's work after the 1950s), and jazz-rock fusion (Don Ellis' and Maynard Ferguson's groups of the 1970s). Not all of them are "swing bands."
Many listeners consider big band to denote an idiom, not just an instrumentation. For them, the strategies of arranging and soloing that were established during the 1930s link all large jazz ensembles more than the different rhythmic and harmonic concepts distinguish those of one era, for example bebop, from those of another, for example those of jazz-rock.
Another important consideration is that journalists and jazz fans of the 1930s and '40s drew distinctions between bands that conveyed the most hard-driving rhythmic qualities and frequent solo improvisations, and those that conveyed less pronounced swing feeling and improvisation. The former were called "swing bands" or "hot bands" (e.g., Count Basie's and Duke Ellington's). The latter were called "sweet bands" (e.g., Glenn Miller's, Wayne King's, Freddy Martin's, and Guy Lombardo's). Although the big-band era ended by 1946, there have been some large orchestras used in jazz ever since, even if virtually none (other than the Count Basie ghost band) operate on a full-time basis. Nearly all are led by arrangers.