Monitor

someone who supervises (an examination)

someone who gives a warning so that a mistake can be avoided

an ironclad vessel built by Federal forces to do battle with the Merrimac

display produced by a device that takes signals and displays them on a television screen or a computer monitor

electronic equipment that is used to check the quality or content of electronic transmissions

a piece of electronic equipment that keeps track of the operation of a system continuously and warns of trouble

any of various large tropical carnivorous lizards of Africa and Asia and Australia

keep tabs on

check, track, or observe by means of a receiver