Noun

• A person or group of persons to keep watch over an area, especially guards or police walking or driving around at regular intervals.
• An automobile, ship or helicopter assigned to keep a watch over an area to maintain security and order.

Verb

• To keep watch over an area by regularly walking or travelling around it.
• To maintain the order and security of an area.
• To pass along a road to maintain the order and security of an area.