monitor

The camp monitors look after the children during the night, when the teachers are asleep.

The information flashed up on the monitor.

a machine code monitor

So, as she did not like the masters to be prying about the play-ground out of school, she chose from among the biggest and most trustworthy of her pupils five monitors, who had authority over the rest of the Boys, and kept the unruly ones in order.

You need not be a monitor to your gracious master the king.