Definitions
1.nounA medicine or other agent to assist hearing.
2.adjectivePertaining to the sense of hearing, the organs of hearing, or the science of sounds; auditory.
3.adjectiveNaturally producing or produced by an instrument without electrical amplification, as an acoustic guitar or acoustic piano.
4.noun(retronym) A hollow-body guitar of the older variety, in contradistinction to an electric guitar.