A small musical group that plays jazz -- typically, a pianist, a base player, a drummer, and one or more "lead" insturments -- a trumpet player, for example, and a saxaphone player.
Combo jazz is improvised, even when it's not: during the head section, when the lead instruments play the melody of the song roughly verbatim, the rhythm section comes up with spontaneous accompanyment; during the solo choruses, in which each player in turn makes up a completely new melody to the chords of the original, the remaining players comp. Thus, because each player must continually respond to those around them, larger groups are unweildy.
The combo has been the typical jazz setting for the past half century, since bebop made its debut. | 
