What does "monitor" mean?

Monitor: Hence, specifically, a pupil selected to look to the school in the absence of the instructor, to notice the absence or faults of the scholars, or to instruct a division or class.

Additional senses

  1. 2.(Zoöl.) Any large Old World lizard of the genus Varanus; esp., the Egyptian species (V. Niloticus), which is useful because it devours the eggs and young of the crocodile. It is sometimes five or six feet long.
  2. 3.Etym: [So called from the name given by Captain Ericson, its designer, to the first ship of the kind.] An ironclad war vessel, very low in the water, and having one or more heavily-armored revolving turrets, carrying heavy guns.
  3. 4.(Mach.) A tool holder, as for a lathe, shaped like a low turret, and capable of being revolved on a vertical pivot so as to bring successively the several tools in holds into proper position for cutting. Monitor top, the raised central portion, or clearstory, of a car roof, having low windows along its sides.

Sources