In my opinion, it is a musical approach rather than a musical genre. It never really settled in my head as a genre. For example, we see air, boards of canada, enigma and sigur ros under the aphex twin in last.fm's ambient tag list of the most popular artists. But as far as I know, air chillout, boards of canada idm, enigma new age, sigur ros make post rock music. These are all ambient music in approach, but technically they belong to roots and genres that should not be confused with each other. I hope I could explain.
dream background music With this music you gently tear the fabric of solid being and get caught between sleep and wakefulness. Reality becomes blurred, matter gets mixed up, perception gets mixed up. You make wings of strange colors and fly, you fall, you die, you cry and smile. it's beautiful, my dear.
It is a music genre that found its place as krautrock-based electronic music in the mid-1970s, not in the 1990s. brian eno.
"ambient music must be able to accommodate many levels of listening attention without enforcing one in particular: it must be as ignorable as it is interesting," brian eno - music for airports, 1978 it should be able to appeal to various levels of listening attention: it should be as negligible as it is interesting."
ambient is a very very very wide and deep genre. oh it's so wide. you'll get lost-we'll be lost