An introduction before a book begins is an example of a prologue.
Riots caused by dissatisfaction with the government may be a prologue to a revolution.
Origin of prologue
From American Heritage Dictionary of the English Language, 5th Edition
- From Old French, from Latin prologus, from Ancient Greek Ï€ÏÏŒÎ»Î¿Î³Î¿Ï‚ (prologos)