Source : Webster's Revised Unabridged Dictionary (1913)

Spy \Spy\, v. i.
   To search narrowly; to scrutinize.

         It is my nature's plague To spy into abuses. --Shak.

Spy \Spy\, v. t. [imp. & p. p. {Spied}; p. pr. & vb. n.
   {Spying}.] [OE. spien, espien, OF. espier, F. ['e]pier, OHG.
   speh?n, G. sp["a]hen; akin to L. specere to see, Skr. spa(?).
   ? 169. Cf. {Espy}, v.t., {Aspect}, {Auspice}, {Circumspect},
   {Conspicuouc}, {Despise}, {Frontispiece}, {Inspect},
   {Prospect}, {Respite}, {Scope}, {Scecimen}, {Spectacle},
   {Specter}, {Speculate}, {Spice}, {Spite}, {Suspicion}.]
   To gain sight of; to discover at a distance, or in a state of
   concealment; to espy; to see.

         One in reading, skipped over all sentences where he
         spied a note of admiration.              --Swift.

   2. To discover by close search or examination.

            Look about with yout eyes; spy what things are to be
            reformed in the church of England.    --Latimer.

   3. To explore; to view; inspect; and examine secretly, as a
      country; -- usually with out.

            Moses sent to spy Jaazer, and they took the villages
            thereof.                              --Num. xxi.
                                                  32.

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}.

Source : WordNet®

spy
     n 1: (military) a secret agent hired by a state to obtain
          information about its enemies or by a business to obtain
          industrial secrets from competitors [syn: {undercover
          agent}]
     2: a secret watcher; someone who secretly watches other people;
        "my spies tell me that you had a good time last night"
     [also: {spied}]

spy
     v 1: catch sight of [syn: {descry}, {spot}, {espy}]
     2: watch, observe, or inquire secretly [syn: {stag}, {snoop}, {sleuth}]
     3: secretly collect sensitive or classified information; engage
        in espionage; "spy for the Russians"
     [also: {spied}]