Sentinel (noun)

1. A person or thing that stands guard and keeps watch over a particular place or thing.
2. A person or thing that serves as a warning or signal of danger or caution.
3. A soldier or guard stationed as a lookout.

Origin:

Derived from the latin word 'sentinēla' meaning a guard or watchman.

Examples:

1. The sentinel at the entrance to the castle was vigilant and watchful, never taking his eyes off the approach.
2. The smoke from the burning building was a sentinel of the fire and danger within.
3. The sentinel tower was strategically placed to give a clear view of the surrounding area, allowing for early detection of any threats.
4. The sentinel system was designed to detect any unauthorized activity and alert the authorities immediately.
5. The sentinel dog was trained to bark at any strangers and provide an early warning to its owners.