Adam's guitar is a futuristic musical instrument used in the 23rd and 24th century.
Adam played a six-stringed instrument, similar to a guitar but without the resonating body. (TOS: "The Way to Eden")
A similar instrument was played by the Edo on Rubicun III and by Katie aboard the USS Enterprise-D; her instrument had eighteen strings, however. (TNG: "Justice", "When The Bough Breaks")