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