As a fan of ambient music, I admit that I've sometimes asked myself the uncomfortable question: is this really any different from Muzak? (If you don't know that word: it's the brand name for the service that provides background music for stores, offices, and so forth. Like "Kleenex", it also serves as the generic term for the kind of product it is.)
I have two answers: "No, not really" and "Yes." It certainly can be and often is used in the same way. Go into a certain type of trendy retail shop, maybe one with a new age spirituality vibe, and you may hear dreamy background music, possibly based on electronics, possibly having some natural sounds mixed in, but in any case pretty unstructured, without much distinctive rhythm or melody: all atmosphere and little structure. It's using a different vocabulary or repertoire from Muzak, which uses (or used to use) sappy instrumental versions of pop songs, but its function as background is much the same.
But nobody sits and listens to Muzak, at least not as such. It isn't meant to be listened to. Whereas people do listen, quite attentively, to ambient music--but not necessarily, or always. The old master, Brian Eno, put it well: ambient music should be "as ignorable as it is interesting." One difference, a pretty big one, between ambient music and mere background music is that the former is very consciously intended to be what it is.
I'm trying to remember now how I came to be acquainted with the music and with the term, and I can't come up with it. I guess it started with what was once called, first positively and then pejoratively, New Age music: mostly instrumental music, usually on the quiet and contemplative side. And then there was a long love affair with the music of the German group Tangerine Dream, which is mostly fairly complex and definitely not ambient, but is almost all electronic, and gave me a taste for the other-worldly quality which electronics can provide. And for many years I listened regularly to the ambient radio program Music From the Hearts of Space (the name is a little cringey to me, but I certainly heard a lot of good music there. I may have picked up the term "ambient music" there.
But that slight misgiving, that question about ambient and muzak, persisted. So I noticed with interest this headline at The Guardian: "Lost in muzak: how ambient music became cool". I of course didn't know it had ever become un-cool. The piece doesn't really address my question except maybe at the very end, with an observation that if you're listening to it closely it isn't ambient. Yes, but you could ignore it. Or half-ignore it. You can sort of step in and out of it at will, and you haven't lost the thread, the plot so to speak, as you would with classical music or for that matter almost any other kind of music. Steven Hill, the proprietor of HOS, uses the word "contemplative" a lot, and that's generally applicable.
Cool or not, there's no doubt that some specimens of the genre remain...interesting, and not ignored. Eno's Music for Airports released in 1978, has as much claim to "classic" status in the general category of non-classical music as anything else of its time. I think its subtitle, Ambient 1, may be the first use of the term in this context. It sounds as fresh to me now as when I first heard it, which was not when it came out but still probably something close to thirty years ago.