Definition of spy

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spy    (sp) KEY 

NOUN:
pl. spies  (spz) KEY 

1. An agent employed by a state to obtain secret information, especially of a military nature, concerning its potential or actual enemies.
2. One employed by a company to obtain confidential information about its competitors.
3. One who secretly keeps watch on another or others.
4. An act of spying.

VERB:
spied  (spd) KEY , spy·ing, spies  (spz) KEY 
VERB:
tr.

1. To observe secretly with hostile intent.
2. To discover by close observation.
3. To catch sight of: spied the ship on the horizon.
4. To investigate intensively.

VERB:
intr.

1. To engage in espionage.
2. To seek or observe something secretly and closely.
3. To make a careful investigation: spying into other people's activities.

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ETYMOLOGY:
Middle English spie, from Old French espie, from espier, to watch, of Germanic origin; see spek- in Indo-European roots