A soldier or guard whose job is to stand and keep watch. The sentinel stood at the gate, vigilant and ready to alert the others of any approaching danger.
A thing that acts as an indicator of the presence of disease. The presence of certain chemicals in the water served as a sentinel for potential contamination.
Computing: a special value placed at the end of a data structure or stream to signal termination or a special condition. A null character is often used as a sentinel to mark the end of a string.