1. The sudden involuntary spasmodic singing of Broadway songs.
(definition from Garrison Keillor's "Guy Noir" February 4, 2006)
2. More broadly, it can also include songs from other musicals, movies, and shows that may or may not have been on Broadway.
"I can't find my keys, but they've got to be somewhere!"
"Somewhere? Somewhere over the rainbow, way up high..."
"Could you control your Broadway Tourette's? I really need to find my keys."