Source : Webster's Revised Unabridged Dictionary (1913)

Prologue \Pro"logue\, n. [F., fr. L. prologus, fr. Gr. ?, fr. ?
   to say beforehand; ? before + ? to say. See {Logic}.]
   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

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

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

Source : WordNet®

prologue
     n : an introduction to a play
     [also: {prologs}, {prologing}, {prologed}]