From Halopedia, the Halo wiki
Overwatch is the state of one small unit or military vehicle supporting another unit, while they are executing fire and movement tactics. An overwatching, or supporting unit has taken a position where it can observe the terrain ahead, especially likely enemy positions. This allows it to provide effective covering fire for the advancing of friendly units. The term overwatch originates in U.S. military doctrine.
UNSC bases station Marines on overwatch, providing visual confirmation of any inbound Covenant ground or air forces, working in tandem with external security cameras, allowing UNSC personnel to respond quickly to the threats.
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