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.
An ability to detect undercurrents and subtleties in a situation.
A device for monitoring the weather.
An ability to predict short-term weather.