Coming from the word ambience, to create an ambience, and it has a few sub-categories, such as:
Ambient dub - Ambient electronica - Ambient house - Ambient groove -
Ambient techno - Ambient trance - and probably more.
Ambient music evolved from early 20th century forms of semi-audible music, from the impressionism
of Erik Satie, through musique concrete and the minimalism of Terry Riley and Philip Glass,
and Brian Eno's deliberate sub-audible approach.
Later developments found the dreamy elements of ambient music applied to some forms of rhythmic music presented in chill-out rooms at raves and other dance events.
The primary feature of Ambient Music is intended to drift in and out of the listener's awareness while
creating its effect on the listener's consciousness.
Ambient dub involves Jamaican sound artists with DJ inspired ambient electronica, complete with all the inherent drop-outs, echo, equalization and psychedelic electronic effects.
Organic Ambient music is characterised by integration of electronic, electric, and acoustic musical instruments, and tends to incorporate influences from world music, especially drone instruments and hand percussion.
Space music, can be called Ambient music as well, as certain characteristics are in common to create the experience of contemplative spaciousness.
Easy Listening and Atmosperic Music may fall into this category too