What is the meaning of Sentinel?

A sentry, watch, or guard.

A private soldier.

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.

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

(Can we verify this sense?) Someone who is hardworking and dutiful.

To watch over as a guard.

To post a guard for.

Source: wiktionary.org