Free Improvisation/Free Jazz

improvised music without any rules beyond the logic or inclination of the musician(s) involved. The term can refer to both a technique (employed by any musician in any genre) and as a recognizable genre in its own right. Conventional songs are highly uncommon in free improvisation; more emphasis is generally placed on mood, texture or more simply, on performative gesture than on preset forms of melody, harmony or rhythm. These elements are improvised at will, as the music progresses. As it has influenced and been influenced by other areas of exploration, aspects of modern classical music (extended techniques), noise rock (aggressive confrontation and dissonance), IDM (computer manipulation and digital synthesis), minimalism and electroacoustic music can now be heard in free improvisation.

Source: http://en.wikipedia.org/wiki/Free_improvisation