noun

• (espionage) A person who secretly watches and examines the actions of other individuals or organizations and gathers information on them (usually to gain an advantage).
  "The dead leap at the throat, destroy The meaning of the day; dark forms Have scaled your walls, and spies betray Old secrets to amorphous swarms."
• (American football) A defensive player assigned to cover an offensive backfield player man-to-man when they are expected to engage in a running play, but the offensive player does not run with the ball immediately.

verb

• (intransitive) To act as a spy.
  "During the Cold War, Russia and America would each spy on each other for recon."
• (transitive) To spot; to catch sight of; to espy.
  "I think I can spy that hot guy coming over here."
• (intransitive) To search narrowly; to scrutinize.
  "(As I confeſſe it is my Natures plague / To ſpy into Abuſes, and of my iealouſie / Shapes faults that are not)"
• (transitive) To explore; to see; to view; inspect and examine secretly, as a country.