“Ambient Music must be able to accomodate many levels of listening attention without enforcing one in particular; it must be as ignorable as it is interesting.” - Brian Eno
Ambient music is a musical genre that is generally identifiable as being broadly atmospheric and environmental in nature. A stronger emphasis is placed upon soundscaping than rhythm or melody.
However, ambient has been fused with other genres of music. Ambient techno fuses atmospheric soundscapes with the rhythm and melodies of techno, and (at times) contain beats. Ambient house keeps the signature four-on-the-floor beats of house with the aesthetics of ambient, using vocal samples and synths in a more "dreamy" fashion. Drone involves a repetition of sounds and long-lasting notes. Black ambient could be considered the metal offshoot of drone.
These are but a handful of examples. Prior to tagging an album, please only keep the fundamental definition in mind. If possible, please specify which subgenre of ambient the album contains in the genre box listing, but simply keep it as "ambient" if you are unsure.