The Collaborative International Dictionary of English v.0.48:

Spy \Spy\ (sp[imac]), v. t. [imp. & p. p. Spied (sp[imac]d);
   p. pr. & vb. n. Spying.] [OE. spien, espien, OF. espier, F.
   ['e]pier, OHG. speh[=o]n, G. sp[aum]hen; akin to L. specere
   to see, Skr. spa[,c]. [root] 169. Cf. Espy, v. t.,
   Aspect, Auspice, Circumspect, Conspicuous, Despise,
   Frontispiece, Inspect, Prospect, Respite, Scope,
   Specimen, Spectacle, Specter, Speculate, Spice,
   Spite, Suspicion.]
   1. To gain sight of; to discover at a distance, or in a state
      of concealment; to espy; to see.
      [1913 Webster]

            One, in reading, skipped over all sentences where he
            spied a note of admiration.           --Swift.
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   2. To discover by close search or examination.
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            Look about with your eyes; spy what things are to be
            reformed in the church of England.    --Latimer.
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   3. To explore; to view, inspect, and examine secretly, as a
      country; -- usually with out.
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            Moses sent to spy out Jaazer, and they took the
            villages thereof.                     --Num. xxi.
                                                  32.
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The Collaborative International Dictionary of English v.0.48:

Spy \Spy\, v. i.
   To search narrowly; to scrutinize.
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         It is my nature's plague
         To spy into abuses.                      --Shak.
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The Collaborative International Dictionary of English v.0.48:

Spy \Spy\, n.; pl. Spies (sp[imac]z). [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.
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   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.
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   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.
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   Syn: See Emissary, and Scout.
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WordNet (r) 3.0 (2006):

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: spy,
         undercover agent]
    2: a secret watcher; someone who secretly watches other people;
       "my spies tell me that you had a good time last night"
    v 1: catch sight of [syn: descry, spot, espy, spy]
    2: watch, observe, or inquire secretly [syn: spy, stag,
       snoop, sleuth]
    3: catch sight of; to perceive with the eyes; "he caught sight
       of the king's men coming over the ridge" [syn: spy,
       sight]
    4: secretly collect sensitive or classified information; engage
       in espionage; "spy for the Russians"

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "spy":
   CIA man, agent, be vigilant, be watchful, beagle, behold, betrayer,
   blab, blabber, blabberer, blabbermouth, bug, case, catch sight of,
   clap eyes on, cloak-and-dagger operative,
   counterintelligence agent, counterspy, delator, descry, detect,
   detective, dig out, discern, discover, distinguish, double agent,
   espionage agent, espy, eye, eyeball, ferret out, fink, fish out,
   follow, gaze at, glimpse, grub up, have a looksee, have in sight,
   hold in view, hunt out, identify, informer, inside man,
   intelligence agent, investigator, keep in sight, keep in view,
   keep under observation, ken, lay eyes on, look after, look at,
   look on, look upon, make a reconnaissance, make out, meddle,
   military-intelligence man, narc, naval-intelligence man, nose,
   notice, observe, operative, peacher, peek, peep, perceive,
   pick out, pick up, play the spy, pry, pry out,
   put under surveillance, recognize, reconnoiter, reconnoiterer,
   regard, root out, scout, scout out, search out, secret agent, see,
   sense, sight, sleuth, snitch, snitcher, snoop, spook, spot,
   spotter, spy out, spy upon, spy-catcher, squealer, stag, stake out,
   stool pigeon, stoolie, take in, talebearer, tattler, tattletale,
   telltale, twig, undercover man, view, watch, whistle-blower,
   witness