The Transition Track is a track of sound of the next scene that starts up at the very end of the previous scene. This track is said to "bleed" into the previous scene.
Required for a Split Edit. Bonus points if the track matches the mood or words at the end of the other scene. For instance, if a character has just been let down, and sad violin music pipes up. Cut to a fancy restaurant where the sad music is being played by the house entertainment.