Adam's guitar.
Edo guitar.
Katie's guitar.
Adam's guitar is a futuristic musical instrument used in the 23rd and 24th Century.
Adam played a 6-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 (TNG: "Justice") and by Katie aboard the USS Enterprise-D, her instrument had 18 strings however. (TNG: "When The Bough Breaks")