The answer to this question is rather simple, but I'm going to give you the reason why.
Answer: a genre
Now lets start at the top, a prelude to a song is in the actual song itself and does not define the song in anyway. If the song has words or a singer, the prelude is most of the music before the words/singer starts. It is a piece of music played as an introduction to the main piece of a song.
Next up and the second most unlikly answer: a dissonance. Like a prelude, this also sometimes happens in a song... as a mistake a decent portion of the time. A dissonance is disagreeable sounds; opposite of the harmony.
A nocturne is an instrumental composition of a pensive, dreamy mood, especially one for the piano, and thats all I have time for before my next class.
Seeya!