This word you write, and say to themselves, perhaps, not very often. "The store has received a big batch of new products"; "Chess player won the game"; "In the elections were won by the democratic Party, " in each of these phrases the word "party" has a different meaning. And you of course, it is clear without explanation.
What about the music? And the word "party" has not one, but two different values.
With one you probably have encountered. During the concert announced: "the piano sings..."; "the part of don Jose in Carmen sang..." and so on. Party in this case is called a part of the musical works performed by one musician or more in unison. So, any string Quartet (we mean the Quartet - the work consists of the parties of the first violin, second violin, viola and cello. In the operas each soloist own party. In romance you hear a lot of voices and a piano. Party basses, tenors, sopranos and Altos and is a choral work.
But you can hear the following sentence: "the Main role of this Sonata I really liked, but side seems not so good". In this case, the word "party" means one of the sections of the Sonata form. More about this is you can read in the stories about the Sonata and on the exposure.