To watch over as a guard.
To post as guard.
To post a guard for.
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 act as a spy.
To search narrowly; to scrutinize.
To explore; to see; to view; inspect and examine secretly, as a country.
To spot; to catch sight of.
A defensive player assigned to cover an offensive backfield player man-to-man when they are expected to engage in a running play, but the offensive player does not run with the ball immediately.
A person who secretly watches and examines the actions of other individuals or organizations and gathers information on them (usually to gain an advantage).