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