A sentry, watch, or guard.
A private soldier.
A unique string of characters recognised by a computer program for processing in a special way; a keyword.
The tag is a sentinel that suspends web-page processing and displays the subsequent text literally.
A sentinel crab.
A sign of a health risk (e.g. a disease, an adverse effect).
sentinel animals can be used to explore endemic diseases.
To watch over as a guard.
He sentineled the north wall.
To post as guard.
He sentineled him on the north wall.
To post a guard for.
He sentineled the north wall with just one man.