prologue

Short definition from Wiktionary

Noun

1. 1
   A speech or section used as an introduction, especially to a play or novel.
2. 2
   One who delivers a prologue.
3. 3
   A component of a computer program that prepares the computer to execute a routine.
4. 4
   An individual time trial before a stage race, used to determine which rider wears the leader's jersey on the first stage.
5. 5
   A liturgical book containing daily readings, including hagiography.

Verb

1. 1
   To introduce with a formal preface, or prologue.