Longman Dictionary of Contemporary English - monitor

~1 v 1 to carefully watch and check a situation in order to see how it changes or progresses over a period of time  (British Aerospace has built a scanner that monitors damage to the ozone layer. | Their job is to monitor healthcare costs.) 2 to secretly listen to other people's phone calls, foreign radio broadcasts etc  (The Security Police had monitored all of his phone calls.) ~2 n 1 a television that shows a picture of what is happening in a particular place  (a row of monitors covering key areas of the building) 2 the part of a computer that looks like a television and that shows information; vdu 3 a piece of equipment that receives and shows information about what is happening inside someone's body  (a monitor that shows the baby's heartbeat) 4 a child who has been chosen to help the teacher in some way  (the milk monitors) 5 someone whose job is to listen to news, messages etc from foreign radio stations and report on them