sentinel

American  

[sen-tn-l] / ˈsɛn tn l /

noun

1. a person or thing that watches or stands as if watching.

   The cats were the sentinels of the house, patrolling constantly for rodents, dogs, and other invaders.

   Synonyms:

   lookout, watch, guard, sentry

2. a soldier stationed as a guard to challenge all comers and prevent a surprise attack.

   Lincoln refused to make his home mansion a garrison during the Civil War, but plain-clothes sentinels did patrol the property

   Synonyms:

   lookout, watch, guard, sentry

3. Digital Technology. tag.

4. Medicine/Medical. an indication or mark that a disease is present or prevalent.

   New viruses in the wastewater can be used as sentinels of future outbreaks.

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verb (used with object)

sentineled, sentineling, sentinelled, sentinelling

1. to watch over or guard as a sentinel.

   This monument sentinels each soldier's grave as a shrine.

adjective

1. Medicine/Medical. relating to or being an indication of a disease's presence or prevalence.

   Pregnant women attending prenatal appointments serve as a sentinel population for the prevalence of malaria in the region.

   The sentinel lymph nodes are the first lymph nodes that the cancer cells reach if they spread.

sentinel British  

/ ˈsɛntɪnəl /

noun

1. a person, such as a sentry, assigned to keep guard

2. computing a character used to indicate the beginning or end of a particular block of information

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verb

1. to guard as a sentinel

2. to post as a sentinel

3. to provide with a sentinel

Other Word Forms

• sentinellike adjective
• sentinelship noun
• unsentineled adjective
• unsentinelled adjective

Etymology

Origin of sentinel

First recorded in 1570–80; from Middle French sentinelle, from Italian sentinella, derivative of Old Italian sentina “vigilance,” from Latin sent(īre) “to feel” + -īna -ine ²

Example Sentences

"When you only have a handful of species that can persist in environments like that, and they're really sensitive to change, those serve as really good sentinel taxa," Adams said.

From Science Daily • Jan. 10, 2026

Ahead of our return to the city, we drive back up the ridge, back through Burguete, the inn still shuttered, and up to Roncesvalles, where the old monastery looms like a sentinel over the pass.

From Salon • Nov. 8, 2025

The church stood empty like a silent sentinel over a small cemetery next to it.

From Slate • Oct. 24, 2024

“This is a sentinel species,” Pozo said, referring to the canary-in-a-coal mine effect where one species can say a lot about an ecosystem.

From Seattle Times • May 21, 2024

This was a wood of stubborn sentinel trees armored in grey-green needles, of mighty oaks, of ironwoods as old as the realm itself.

From "A Game of Thrones" by George R.R. Martin