Guard

a person who keeps watch over something or someone

the person who plays that position on a football team

a device designed to prevent injury or accidents

a posture of defence in boxing or fencing

the person who plays the position of guard on a basketball team

a military unit serving to protect some place or person

a precautionary measure warding off impending danger or damage or injury etc.

the duty of serving as a sentry

(American football) a position on the line of scrimmage

a position on a basketball team

to keep watch over

watch over or shield from danger or harm

protect against a challenge or attack

take precautions in order to avoid some unwanted consequence