1. a guard who keeps watch, especially a soldier stationed to keep guard
2. something that serves as a warning or indicator

1. to watch over as a sentinel; to stand guard

Examples

• The sentinel stood motionless at his post throughout the night.
• Ancient towers served as sentinels overlooking the valley.
• The lighthouse acted as a sentinel, warning ships of dangerous rocks.
• A lone pine tree sentineled the mountain peak.
• The guard dog sentineled the entrance to the warehouse.
• Sentinels were posted at every corner of the fortress.
• The old oak tree stood like a sentinel in the empty field.