I run a folk club that describes itself as an "acoustic music venue". All that means is that I do not allow electronic or electric instruments. That is to say, instruments whose method of sound reproduction requires electricity. This rules out, eg, electric guitars and keyboards. There is no problem amplifying guitars, voice or other string or wind instruments using microphones, because the amplification is only sound reinforcement. The instrument is still "acoustic".
It is about how the sound is produced. Acoustically or electronically.