n. The way that a good majority of mainstream hip-hop songs get their melodies. Technically, it's a form of sampling; an interpolation is basically borrowing (or stealing) the melody of another song and recording it as your own. In theory, it's supposed to be done legally, although there have been cases of artists reusing melodies without permission, which have led to lawsuits.
Girl Singing "Changes": "That's just the way it is! Things'll never be the same!" That's my favorite 2pac song!
Music Snob: 2pac didn't write that! Bruce Hornsby did! It's an "interpolation"!
*Girl smacks Music Snob*