A sentry, watch, or guard.
A unique string of characters recognised by a computer program for processing in a special way; a keyword.
A sentinel crab.
A sign of a health risk (e.g. a disease, an adverse effect).
To post as guard.
To post a guard for.
To watch over as a guard.
A sentinel, usually on horseback, stationed on the outpost of an army, to watch an enemy and give notice of danger.
A cabaret performer, usually the main female artist of a show.