Source : Webster's Revised Unabridged Dictionary (1913)

Monitor \Mon"i*tor\, n. [L., fr. monere. See {Monition}, and cf.
   {Mentor}.]
   1. One who admonishes; one who warns of faults, informs of
      duty, or gives advice and instruction by way of reproof or
      caution.

            You need not be a monitor to the king. --Bacon.

   2. 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.

   3. (Zo["o]l.) 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.

   4. [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.

   5. (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.