Eine kleine Nachtmusik was a piece of classical music written by Earth composer Wolfgang Amadeus Mozart in the late 18th century.
A crew member of the USS Enterprise-D fantasized playing the first movement (Allegro) of Eine kleine Nachtmusik together with three other musicians in period clothing, when he was affected by being in a far away galaxy where thoughts became reality in 2364. (TNG: "Where No One Has Gone Before")
A string quartet consisting of Data, playing the violin, Miles O'Brien, playing the cello and two more crewmembers of the Enterprise-D (playing the violin and viola) performed the first movement of Eine kleine Nachtmusik at the beginning of the year 2366. (TNG: "The Ensigns of Command")