A sentry, watch, or guard.
A unique string of characters recognised by a computer program for processing in a special way; a keyword.
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.
A period of observation or surveillance at any hour.
A quiet demonstration in support of a cause.
An instance of keeping awake during normal sleeping hours, especially to keep watch or pray.
The eve of a religious festival in which staying awake is part of the ritual devotions.