If anyone cares to come to the Greyhound in Penkhull, Stoke-on-Trent, on Wednesday night, Dec 17, you will hear what I personally mean by acoustic music. You will hear the Boat Band playing acoustically, no mics, no PA, in the bar,(not a back room, folk clubby sort of thing, by the way).
Now, I know this meaning of the word acoustic(ie unamplified)is disputed by many. As we see on this thread, many people use it to mean only a bit amplified. Or no electric guitars, but maybe an electic bass and keyboards. I would use that looser meaning myself sometimes, in the phrase "they've gone for an acoustic sound on this tour", or some phrase like that. But I still think, basically, that "acoustic"(in the context of live music) means "unamplified": the music goes from the instrument to my ear by acoustic means, unprocessed electrically.
It's all much like the meaning of folk. There are still people using the old meaning of acoustic, but usage is gradually changed to the new.Sadly, in my opinion,it is a perfectly good word as it stands. But I am resigned to the fact that "acoustic night" at the local pub will be nothing of the sort.