A guitar that does not rely on electronic amplification to project its sound. An acoustic guitar features a resonating chamber (its body) that amplifies the vibrations from the strings to provide sufficient volume for the sound to reach listeners. Acoustic guitars do not include onboard electronics or a pickup, but make their sound entirely in the acoustic domain — strings and body vibrating and resonating to create sound in the air.
In general parlance, the term “acoustic guitar” is often used broadly enough to include guitars with pickups and electronics, though technically, such a guitar would be an “acoustic-electric guitar.”