Through the fog they could hear the mournful, monitory note of the foghorn. A professor may start class with a monitory comment about final exams. A president may make a monitory speech addressed to a country that is ignoring its trade obligations. And a pope may issue a monitory message to the world on the subject of war or morality. The North's armored ship the Monitor was designed to monitor the South's naval activities in the coastal waters.
Monitor can be both a verb and a noun.
A heart monitor monitors a patient's heartbeat and warns of any problems. A hall monitor monitors students behavior in the hallways. Both machine and human monitors observe or supervise and give warnings if something goes wrong. A feeling about an event or situation before it happens. He now remembered how the birds had been restless and noisy, as though they had felt a premonition of the coming earthquake.