Yes, MONITOR is a Scrabble word!

Letter values in Scrabble US: M 3, O 1, N 1, I 1, T 1, O 1, R 1.

Is it a valid word?

MONITOR is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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; fabled to warn of crocodiles

Also: monitoring device, monitor lizard.

Not playable in Scrabble US: VARAN.