Meaning

• 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."

Meaning

• 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."