monitor has 7 distinct synonyms spread across 5 of its 9 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
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; fabled to warn of crocodiles
check, track, or observe by means of a receiver
Synonyms, antonyms and the broader and narrower relations for monitor come from Princeton University’s WordNet. Full sources and licences.