sentinel
nounUK /ˈsɛntɪnəl/
US /ˈsɛntənəl/
A sentinel is a person or animal that watches over a place to keep it safe.
A soldier stood as a sentinel at the gate all night.
RELATED WORDS
OTHER MEANINGS
2. A soldier whose job is to stand and watch for danger.
The sentinel called out when he saw strangers approaching.
3. Something that stands watch or marks a place, like a tall stone or a special tree.
An old oak tree stood like a sentinel at the edge of the field.
4. A computer program that keeps checking something and warns if there is a problem.
The sentinel program alerted the team when the server stopped working.