The Piano Sonata No. 14 in C-sharp minor "Quasi una fantasia", Op. 27 , No. 2, popularly known as the Moonlight Sonata, is a piano sonata by Ludwig van Beethoven. Completed in 1801 and dedicated in 1802 to his pupil, Countess Giulietta Guicciardi, it is one of Beethoven's most popular compositions for the piano.
The first edition of the score is headed Sonata quasi una fantasia, a title this work shares with its companion piece, Op. 27, No. 1. Grove Music Online translates the Italian title as "sonata in the manner of a fantasy". Translated more literally, this is "sonata almost a fantasy".
The name "Moonlight Sonata" comes from remarks made by the German music critic and poet Ludwig Rellstab. In 1832, five years after Beethoven's death, Rellstab likened the effect of the first movement to that of moonlight shining upon Lake Lucerne. Within ten years, the name "Moonlight Sonata" ("Mondscheinsonate" in German) was being used in German and English publications. Later in the nineteenth century, the sonata was universally known by that name.