A person who secretly gathers information for a group or state.

• Someone who watches others covertly to gain advantage.

In American football, a defender assigned to shadow a mobile backfield player, often the quarterback.

• A player who monitors and contains a specific runner on potential scrambles.

To act as a spy and gather information covertly.

• To secretly watch someone or something for intelligence.

To notice or catch sight of, especially after searching.

• To spot something unexpectedly or from a distance.