sentinel

noun

1. A sentry, watch, or guard.

   “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.”

   “the sentinels who paced the ramparts”

   “that princes do keep due sentinel”

2. (obsolete)A private soldier.

   ““I will not permit the poorest centinel to be treated with injustice.””

3. A unique value recognised by a computer program for processing in a special way, or marking the end of a set of data.

   “The <xmp> tag is a sentinel that suspends web-page processing and displays the subsequent text literally”

   “[…] a sentinel value that indicates a missing entry.”

4. A sentinel crab.
5. (attributive)A sign of a health risk (e.g. a disease, an adverse effect).

   “sentinel animals can be used to explore endemic diseases.”

verb

1. (transitive)To watch over as a guard.

   “He sentineled the north wall.”

2. (transitive)To post a guard for.

   “He sentineled the north wall with just one man.”

   “The old-fashioned stoop, with its suggestive benches on either side, lay solitary and silent in the moonlight; the garden path, weedily overgrown since father's death, and sentineled here and there with ragged hollyhock, lay quiet and dew-laden […]”

name

1. A place in the United States:
2. A place in the United States:
3. A place in the United States:
4. A place in the United States: