Sentinel

n.	1.	(Mil.) One who watches or guards; specifically (Mil.), a soldier set to guard an army, camp, or other place, from surprise, to observe the approach of danger, and give notice of it; a sentry. The sentinels who paced the ramparts. - Macaulay.
	2.	Watch; guard.
	3.	(Zool.) A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; - called also sentinel crab.
v. t.	1.	To watch over like a sentinel. [imp. & p. p. Sentineled or Sentinelled; p. pr. & vb. n. Sentineling or Sentinelling.]
	2.	To furnish with a sentinel; to place under the guard of a sentinel or sentinels.

Noun

1.

sentinel - a person employed to watch for something to happen Synonyms: lookout, lookout man, picket, scout, sentry, spotter, watch

lookout, lookout man, picket, scout, security guard, sentry, spotter, watch, watcher, watchman