sentinel
UK: ˈsɛntɪn(ə)l | US: ˈsɛn(t)ən(ə)l
n. 1. A guard or watchperson stationed to protect or observe.
n. 2. (Computing) A monitoring system or process that detects changes or threats.
n. 3. (Figurative) Something that serves as a warning or indicator.
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The word "sentinel" traces back to Latin sentire (to feel, perceive), reflecting the guard’s role in vigilance. The suffix -inel likely evolved via Old French, adding a diminutive or agentive nuance (similar to -el in "colonel"). The term entered English in the 16th century, originally denoting a soldier assigned to watch. Over time, it expanded metaphorically to include symbolic "watchers" (e.g., sentinel plants in ecology).
The ancient castle had a sentinel posted at every tower.
Cybersecurity systems use sentinels to detect intrusions.
The lone tree stood like a sentinel over the barren landscape.
Elevated white blood cells act as sentinels of infection.
The satellite serves as a sentinel for climate monitoring.