spy

One who secretly collects information concerning the enemies of a government or group.

One who secretly collects information for a business about one or more of its competitors.

One who secretly keeps watch on another or others.

To watch or observe secretly.

To discover by close observation.

To catch sight of; see.

To engage in espionage.

To investigate or observe something, especially in secret.

To discover at a distance, or from a position of concealment; gain sight of; see; espy.

To discover by close search or examination; gain a knowledge of by artifice.