Definitions:
a device used for observing, checking, or keeping a continuous record of something

a television receiver used in a studio to select or verify the picture being broadcast from a particular camera

a school pupil with disciplinary or other special duties

a large tropical old world lizard with a long neck, narrow head, forked tongue, strong claws, and a short body. monitors were formerly believed to give warning of crocodiles.

a shallow-draught warship mounting one or two heavy guns for bombardment.

observe and check the progress or quality of (something) over a period of time; keep under systematic review