A definition of acoustic music is rather simple: it's music without electric-powered instruments. In the words of wikipedia.org:

Acoustic music solely or primarily uses acoustic instruments, such as the acoustic guitar, banjo, mandolin, piano, cello, and voice. The category includes much contemporary folk and singer-songwriter music, as well as bluegrass music. It can also refer to instrumental music that is created using acoustic instruments.
(The above was contributed by Raine.)

For additional acoustic music content, see: