Hi Pineapple. Often times in creative forms of expression such as music, art and dance, one "work', routine or song would be referred to as a "piece". My guess is that it's called a piece because it is a "portion" of something bigger / greater such as a collection. For example, an artist may have a large collection of paintings, each one an individual, or "piece" of the larger collection. The best painting in the artist's collection would be considered the "Masterpiece". hope this helps