Ambient means the natural center or atmosphere of a space. All music has that in it- a space or center. I think it just means the atmosphere or what defines the environment of sound and maybe removing the more destructive, harsh elements and harder rhythmic elements and you get down to the stillness that's inherent. There's an ambient quality in every sound. You may have to enhance that to hear it or bring it out in a different way but there is that in every environmental sound.