From BiblePortal Wikipedia
Webster's Dictionary 
(1): ( v. t.) To introduce with a formal preface, or prologue.
(2): ( n.) 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
(3): ( n.) One who delivers a prologue.
International Standard Bible Encyclopedia 
prō´log , prol´og ( πρόλογος , prólogos , "foreword," "preface," "introduction"): The word occurs in the preface to Ecclesiasticus (Sirach), and is commonly applied to John 1:1-18 . See Ecclesiasticus; John , Gospel Of .