11 o'clock number is a theatrical term for a big, show-stopping song that occurs late in the second act of a two-act musical. It was so named because in the days when musical performances would start at 8:30 pm, this song would occur around 11 pm. Besides reenergizing the audience as the show heads toward its conclusion, it offers audience members a catchy melody to hum as they leave the theater.
Among the theatre community, there is some debate as to additional characteristics of an 11 o'clock number. It often signifies a moment of revelation or change in heart of a lead character, although there are exceptions to this. The 11 o'clock number is also differentiated from the finale in that it is not the final number in the show, but even this is not considered a requirement by some commenters.
Notable 11 o'clock numbers include "Sit Down, You're Rockin' the Boat" from Guys and Dolls, "Rose's Turn" from Gypsy, "Brotherhood of Man" from How to Succeed in Business Without Really Trying, "Memory" from Cats, "Gimme Gimme" from Thoroughly Modern Millie, "The American Dream" from Miss Saigon and, more recently, "I'm Here" from The Color Purple, "Always Starting Over" from If/Then and "She Used to Be Mine" from Waitress.