means A monitor is a screen for displaying a computer's output, or more broadly a person or device that watches, checks, and warns about something.

from Straight from Latin 'monitor,' meaning 'one who warns or advises,' from 'monere,' to warn or remind — the same root behind 'admonish' and 'premonition.' For centuries a monitor was a human warner: a senior schoolchild keeping order, a naval warship, a watchful overseer. Only in the 20th century did the word slide onto the glowing screen that now keeps watch over us — the warner became a window, still quietly judging.