Definitions:
watch over in order to protect or control

protect against damage or harm

a person who keeps watch, especially a soldier or other person assigned to protect a person or to control access to a place

a device worn or fitted to prevent injury or damage

a defensive posture that is adopted in a boxing or martial arts contest or in a fight

an official who rides on and is in general charge of a train.

each of two players either side of the centre.