The word monitor, depending on context, may have one of the following meanings:

• A computer display
• A speaker used on stage or in a studio to enable musicians to hear what is being recorded or broadcasted
• An operating system mechanism invented by C.A.R. Hoare that synchronizes operations on shared data by multiple processes
• A hardware device that measures electrical events such as pulses or voltage levels in a digital computer.
• A small interactive bootstrap program or machine code debugger
• An execution profiler
• An early term for an operating system
• USS Monitor, the ironclad warship of the American civil war.
• A monitor warship type of ship based on the USS Monitor and built by several navies for coastal defence in the 1860s and 1870s. It reappeared in a different form during the First World War and lasted unitl the end of the Second World War
• A river monitor, the strongest type of river warships.
• Monitor lizards are a family of large tropical lizards (Varanidae).
• A popular NBC radio program which aired from 1955 to 1975.
• Two characters from the DC Comics limited comic book series, Crisis on Infinite Earths.
• A baby monitor
• Also the AI that is the caretaker of Halo (better known as 343 Guilty Spark).