The Collaborative International Dictionary of English v.0.48:

Prologue \Pro"logue\, v. t. [imp. & p. p. Prologued; p. pr. &
   vb. n. Prologuing.]
   To introduce with a formal preface, or prologue. [R.] --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Prologue \Pro"logue\, n. [F., fr. L. prologus, fr. Gr. ?, fr. ?
   to say beforehand; ? before + ? to say. See Logic.]
   [1913 Webster]
   1. The preface or introduction to a discourse, poem, or
      performance; as, the prologue of Chaucer's "Canterbury
      Tales;" esp., a discourse or poem spoken before a dramatic
      performance
      [1913 Webster]

   2. One who delivers a prologue. [R.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prologue
    n 1: an introduction to a play

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "prologue":
   act, afterpiece, avant-propos, bit, breakthrough, chaser, curtain,
   curtain call, curtain raiser, divertimento, divertissement,
   epilogue, exode, exodus, exordium, expository scene, finale,
   foreword, front matter, frontispiece, hoke act, innovation,
   interlude, intermezzo, intermission, introduction, leap, number,
   overture, postulate, preamble, preface, prefix, prefixture,
   preliminary, prelude, premise, presupposition, proem, prolegomena,
   prolegomenon, prolepsis, protasis, routine, scene, shtick, sketch,
   skit, song and dance, stand-up comedy act, striptease, turn, verse,
   voluntary