Diegetic Song
A song that exists within the world of the story—the characters know they are singing, and there's a realistic reason for the music (a nightclub performance, a rehearsal, a character sitting at a piano). This contrasts with non-diegetic songs, where characters burst into song as an expression of emotion without any in-world justification. Cabaret is built almost entirely around diegetic songs performed at the Kit Kat Club.
