What is a monitor?

Answer:

display produced by a device that take signals and displays it
A monitor is what takes computer writing from your computer and translates it to audible signals you can see. A monitor is required on every computer that you intend on interacting with visually. There are CRT monitors (Cathose Ray Tube), LCD (Liquid Crystal Display), and presently Plasma. CRT monitors are the older style bulky and cumbersome monitors. LCD is fairly alien, usually very flat and deeply light. Plasma is the up-to-the-minute, flat, but not as light as LCD's. Hope any of this help.
I assume your question refers to an animal and not your computer peak? Monitor lizards are a group of lizards, which also include the largest living lizard, the komodo dragon.
A monitor is a speaker that sits on the front of the stage and faces the singer/musician so they can hear themselves because they cannot clearly hear the speakers on the side of the stage.