Definition of "centinel"

Enter Barnardo, and Franciſco, two Centinels.

c. 1599–1602 (date written), William Shakespeare, “The Tragedie of Hamlet, Prince of Denmarke”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act I, scene i]

When the centinel first saw our heroe approach, his hair began gently to lift up his grenadier cap; and in the same instant his knees fell to blows with each other.

1749, Henry Fielding, The History of Tom Jones, a Foundling, volumes (please specify |volume=I to VI), London: A[ndrew] Millar, […]

To counterbalance those foreign centinels, there mounted guard on the other side of the mirror two stout warders of Scottish lineage; […]

1819, Jedadiah Cleishbotham [pseudonym; Walter Scott], Tales of My Landlord, Third Series. […], volume II (The Bride of Lammermoor), Edinburgh: […] [James Ballantyne and Co.] for Archibald Constable and Co.; London: Longman, Hurst, Rees, Orme, and Brown, […]; Hurst, Robinson, and Co. […], page 301