Noun

1. Someone who supervises (an examination)
2. Someone who gives a warning so that a mistake can be avoided
3. An ironclad vessel built by federal forces to do battle with the merrimac
4. Display produced by a device that takes signals and displays them on a television screen or a computer monitor
5. Electronic equipment that is used to check the quality or content of electronic transmissions
6. A piece of electronic equipment that keeps track of the operation of a system continuously and warns of trouble
7. Any of various large tropical carnivorous lizards of africa and asia and australia; fabled to warn of crocodiles

Verb

1. Keep tabs on; keep an eye on; keep under surveillance
2. Check, track, or observe by means of a receiver