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.-watch; guard.-a marine crab (podophthalmus vigil) native of the indian ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab.-to watch over like a sentinel.-to furnish with a sentinel; to place under the guard of a sentinel or sentinels.