acoustic guitar

n. modern steel-string guitar similar to a classical guitar; sound is not amplified by electrical means

An acoustic guitar is a guitar that uses only acoustic methods to project the sound produced by its strings. It is a retronym, coined after the advent of electric guitars, which depend on electronic amplification to make their sound audible.
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A steel-string acoustic guitar, is a modern form of guitar descended from the classical guitar, but strung with steel strings for a brighter, louder sound. Strictly speaking, the terms steel-stringed guitar, classical guitar, and folk guitar all refer to acoustic (that is, non-electric) guitars, though some of these terms refer to different types of instruments (nylon-strung vs. steel-strung). The term "acoustic guitar" is a retronym, since before the invention of the electric instrument, all guitars were "acoustic."
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Noun
1. sound is not amplified by electrical means
(hypernym) guitar