The amount of time, at a live performance by a popular band, between the beginning of a song and the beginning of the wave of applause signifying that the audience has realised which song is being played.
A: Man, the Perceptisonic Lag on that song was huge... Did those idiots not know what they were listening to?
B: They did play it pretty different from the album version...
A: Yeah, I guess so. But I knew what it was.