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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   Syn: See Emissary, and Scout.
        [1913 Webster]

Easton's 1897 Bible Dictionary:

Spies
   When the Israelites reached Kadesh for the first time, and were
   encamped there, Moses selected twelve spies from among the
   chiefs of the divisions of the tribes, and sent them forth to
   spy the land of Canaan (Num. 13), and to bring back to him a
   report of its actual condition. They at once proceeded on their
   important errand, and went through the land as far north as the
   district round Lake Merom. After about six weeks' absence they
   returned. Their report was very discouraging, and the people
   were greatly alarmed, and in a rebellious spirit proposed to
   elect a new leader and return to Egypt. Only two of the spies,
   Caleb and Joshua, showed themselves on this occasion
   stout-hearted and faithful. All their appeals and remonstrances
   were in vain. Moses announced that as a punishment for their
   rebellion they must now wander in the wilderness till a new
   generation should arise which would go up and posses the land.
   The spies had been forty days absent on their expedition, and
   for each day the Israelites were to be wanderers for a year in
   the desert. (See ESHCOL.)

     Two spies were sent by Joshua "secretly" i.e., unknown to the
   people (Josh. 2:1), "to view the land and Jericho" after the
   death of Moses, and just before the tribes under his leadership
   were about to cross the Jordan. They learned from Rahab (q.v.),
   in whose house they found a hiding-place, that terror had fallen
   on all the inhabitants of the land because of the great things
   they had heard that Jehovah had done for them (Ex. 15:14-16;
   comp. 23:27; Deut. 2:25; 11:25). As the result of their mission
   they reported: "Truly Jehovah hath delivered into our hands all
   the land; for even all the inhabitants of the country do faint
   because of us."