Noun

1. A sentry or guard.
   • 1719- Daniel Defoe, Robinson Crusoe

     They promised faithfully to bear their confinement with patience, and were very thankful that they had such good usage as to have provisions and light left them; for Friday gave them candles (such as we made ourselves) for their comfort; and they did not know but that he stood sentinel over them at the entrance.

2. a unique string of characters recognised by a computer program for processing in a special way; a keyword.

   The <nowiki> tag is a sentinel that suspends web-page processing and displays the subsequent text literally.

Verb

1. To watch over as a guard.

   He sentineled the north wall.

2. To post as guard.

   He sentineled him on the north wall.

3. To post a guard for.

   He sentineled the north wall with just one man.