A device capable of registering a specific substance or physical phenomenon, and that optionally sounds an alarm or triggers a warning.
An indicator showing the depth of the water in a boiler.
A galvanometer, usually portable, for indicating the direction of a current.
A sign of a health risk (e.g. a disease, an adverse effect).
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.
To post as guard.
To post a guard for.
To watch over as a guard.