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.

sentinel A datum that indicates some important state, usually in the context of input or output. For example, an end-of-data sentinel means all the data has been read. See also rogue value, flag.

sentinel sentry; †duty of a sentry; †military watch-tower XVI; †(in full private s.) private soldier XVIII. — F. sentinelle — It. sentinella, of unkn. orig.
Hence vb. XVI.