Pronunciation: (sen'tn-l),[key] —n., v.,-neled, -nel•ing or (esp. Brit.)-nelled, -nel•ling.
—n. 1. a person or thing that watches or stands as if watching.
2. a soldier stationed as a guard to challenge all comers and prevent a surprise attack: to stand sentinel. 3. Also called tag.Computers.a symbol, mark, or other labeling device indicating the beginning or end of a unit of information.