If I can just add a Grumpy Woman's comments, I'd like to say I agree completely with Grumpy Greg. When I see something described as an "acoustic" night, I would (logically) expect this to mean there's no PA system, no electrics involved at all, but it seems to mean almost the opposite.

But I think I see why there is confusion. I think it may be that whereas folkies are used to unamplified music and song, those who are used to more mainstream pop music are used to backing tracks, sampling, dubbing, and all the other tricksy stuff that goes on. When they say "acoustic", they seem to mean that the sounds you'll hear are actually emanating from the singers or instruments present in the room, as opposed to pre-recorded or pre-set keyboard tracks, recorded backing vocals, etc.

So from the pop point of view, "acoustic" means a more authentic, live performance, whereas to those used to the folk scene, it's not "acoustic" if it's amplified. Maybe we just need to drop the word and find new ways of describing what we mean.

The whole issue of whether amplification is a good idea or not is another matter entirely, and what's accepted as normal seems to be different in the US and UK. I will just add one observation: in my experience, as soon as you get amplification in an informal venue, the audience noise level soars, and they become a lot less engaged with the music - the PA system seems to create a barrier between the audience and the music and make them less attentive.