Definition of "sentinel"

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.

1719 May 6 (Gregorian calendar), [Daniel Defoe], The Life and Strange Surprizing Adventures of Robinson Crusoe, […], 3rd edition, London: […] W[illiam] Taylor […], published 1719

the sentinels who paced the ramparts

1849–1861, Thomas Babington Macaulay, chapter 12, in The History of England from the Accession of James the Second, volumes (please specify |volume=I to V), London: Longman, Brown, Green, and Longmans

that princes do keep due sentinel

1625, Francis [Bacon], “Of Empire”, in The Essayes […], 3rd edition, London: […] Iohn Haviland for Hanna Barret

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

1789, John Moore, Zeluco, Valancourt, published 2008, page 33

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

2016, Jake VanderPlas, Python Data Science Handbook, page 120

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 […]

1873, Harper's New Monthly Magazine, volume 46, page 562