A lookout (vantage point with a view of the surrounding area)
A place where stars, planets and other celestial bodies are observed, usually through a telescope; also place for observing meteorological or other natural phenomena.
A unique string of characters recognised by a computer program for processing in a special way; a keyword.
A sentry, watch, or guard.
A sentinel crab.
A sign of a health risk (e.g. a disease, an adverse effect).
To post as guard.
To post a guard for.
To watch over as a guard.