What do you call a group of three to four jazz musicians who play improvised jazz?

Jazz improvisation is the spontaneous invention of melodic solo lines or accompaniment parts. It is one of the defining elements of jazz. Improvisation is composing on the spot, when a singer or instrumentalist invents melodies and lines over a chord progression played by rhythm section instruments (piano, guitar, double bass) and accompanied by drums. Although blues, rock, and other genres use improvisation, it is done over relatively simple chord progressions which often remain in one key (or closely related keys using the circle of fifths, such as a song in C Major modulating to G Major).