1. A piece of writing at the beginning of a book that introduces the story.
2. The beginning, or introductory passage, of a play, a movie, or a television program which introduces the contents.
3. An actor who speaks introductory lines to a dramatic performance before the main action starts.
4. An event or action that leads to something more important: "The husband's affair was a prologue to the complete breakdown of the couple's marriage."
5. Etymology: from Old French prologue (early 13th century), from Latin prologus, from Greek prologos, "prologue of a play, a speaker of a prologue"; literally, "a speech beforehand"; from pro-. "before" + logos, "discourse, speech" which came from legein, "to speak".