Available Definitions:
1) n. - 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.
2) n. - Watch; guard.
3) n. - A marine crab (Podophthalmus vigil) native of the Indian Ocean, remarkable for the great length of its eyestalks; -- called also sentinel crab.
4) v. t. - To watch over like a sentinel.
5) v. t. - To furnish with a sentinel; to place under the guard of a sentinel or sentinels.