Definition of "prologue"

“H'm !” he said, “so, so—it is a tragedy in a prologue and three acts. I am going down this afternoon to see the curtain fall for the third time on what [...] will prove a good burlesque ; but it all began dramatically enough. It was last Saturday […] that two boys, playing in the little spinney just outside Wembley Park Station, came across three large parcels done up in American cloth. […]”

1905, Baroness Emmuska Orczy, chapter 2, in The Lisson Grove Mystery‎

And hither am I come, / A Prologue arm’d, but not in confidence / Of Authors pen, or Actors voyce;

c. 1602, William Shakespeare, “The Tragedie of Troylus and Cressida”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, “the prologue”

[…] harbindgers preceading ſtill the fatesand prologue to the Omen comming on […]

c. 1599–1602 (date written), William Shakespeare, The Tragicall Historie of Hamlet, Prince of Denmarke: […] (Second Quarto), London: […] I[ames] R[oberts] for N[icholas] L[ing] […], published 1604, [Act I, scene i]