prologue (n.)

early 14c., prologe, "introduction to a narrative or discourse," from Old French prologue (12c.) and directly from Latin prologus, from Greek prologos "preface to a play, speaker of a prologue," etymologically "a speech beforehand," from pro "before" (see pro-) + logos "discourse, speech," from legein "to speak," from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')." Especially a discourse or poem spoken before a dramatic performance or play. Figuratively, "a preliminary act or event," by 1590s.