Monitor

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

any of various large tropical carnivorous lizards of Africa and Asia and Australia; fabled to warn of crocodiles

someone who supervises (an examination)

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

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

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

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

keep tabs on; keep an eye on; keep under surveillance

check, track, or observe by means of a receiver