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

a heart monitor

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

the playback can be displayed on an external television screen or monitor

a school pupil with disciplinary or other special duties

he was a school monitor and a choir boy

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

equipment was installed to monitor air quality