A ghost town in California.

A town in Oklahoma.

A sentry, watch, or guard.

(obsolete) A private soldier.

(computer science) A unique value recognised by a computer program for processing in a special way, or marking the end of a set of data.

A sentinel crab.

(attributive, medicine, epidemiology) A sign of a health risk (e.g. a disease, an adverse effect).

(figuratively) Someone who is hardworking and dutiful.

(transitive) To watch over as a guard.

(transitive) To post a guard for.