The story
So called because, in the days when musical performances began at 8:30 p.m., such a song would occur around 11 p.m.
Etymology from Wiktionary, CC BY-SA 4.0.
11 o'clock number
Today it means: A showstopping song that occurs late in the second act of a two-act musical, often signifying a moment of revelation or change of heart of a lead character. Full definition →