PROLOGUE = (from the Greek – 'before speech'): the opening section of a work; a kind of introduction which is part of the work and not prefatory. It was common in drama in the 17th and 18th c., when it was often in verse. Occasionally found in novels. In plays the prologue is usually a chorus. The most famous example in English literature is Chaucer's General Prologue to The Canterbury Tales.

Source: Penguin Dictionary of Literary Terms and Literary Theory by J.A.Cuddon, 1976, 3rd edition.