sen·ti·nel / ˈsentn-əl/ • n. a soldier or guard whose job is to stand and keep watch. ∎ fig. something that appears to be standing guard or keeping watch. ∎ Med. a thing that acts as an indicator of the presence of disease: [as adj.] the first national HIV sentinel surveillance program in the developing world. • v. (-neled, -nel·ing; chiefly Brit. -nelled, -nel·ling) [tr.] station a soldier or guard by (a place) to keep watch: a wide course had been roped off and sentineled with police | fig. trees sentineled the trenches. PHRASES: stand sentinel (of a soldier) keep watch: soldiers stood sentinel with their muskets | fig. a tall round tower standing sentinel over the river.