What is the meaning of letter u in music?

The letter U in music is used to indicate a measure of tempo, or speed. It is an abbreviation of the Italian word "unity," and is used to tell the performer to play at a brisk, moderate pace. The tempo of a piece of music is usually marked by a number, such as 120 or 80, which indicates the number of beats per minute. The letter U is equivalent to a tempo of 120 beats per minute.