Source : Webster's Revised Unabridged Dictionary (1913)

Spy \Spy\, n.; pl. {Spies}. [See {Spy}, v., and cf. {Espy}, n.]
   1. One who keeps a constant watch of the conduct of others.
      ``These wretched spies of wit.'' --Dryden.

   2. (Mil.) A person sent secretly into an enemy's camp,
      territory, or fortifications, to inspect his works,
      ascertain his strength, movements, or designs, and to
      communicate such intelligence to the proper officer.

   {Spy money}, money paid to a spy; the reward for private or
      secret intelligence regarding the enemy.

   {Spy Wednesday} (Eccl.), the Wednesday immediately preceding
      the festival of Easter; -- so called in allusion to the
      betrayal of Christ by Judas Iscariot.

   Syn: See {Emissary}, and {Scout}.