What is ambient music ?

What is ambient music ? A friend of mind asked me this question the other day and it struck me as odd that I couldn't find a definition that I found satisfactory for my own mind. In other words, I wasn't happy because I found myself saying "it's this ...but it's also this...it's not this nor this but then again it could be if ... ". So I've decided to open this question up to all psyamb listeners ... what is your own definition of ambient music ?

Is it , as one of the most recognized of ambient artists Brian Eno said, music that can be either "actively listened to with attention or as easily ignored, depending on the choice of the listener", and that exists on the "cusp between melody and texture" ? I kind of like this definition myself. That line "cusp between melody and texture" evokes an ambient music feel.

With so many sub-genres of ambient music these days including dark ambient, illbient, ambient techno, ambient house, isolational, nature ambient and organic soundscapes is it even possible to come up with an overall answer for what is ambient music ? Drop a comment below and let me know what you think.