The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            You need not be a monitor to the king. --Bacon.
      [1913 Webster]

   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.
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   3. (Zool.) Any large Old World lizard of the genus Varanus;
      esp., the Egyptian species (Varanus Niloticus), which is
      useful because it devours the eggs and young of the
      crocodile. It is sometimes five or six feet long.
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   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.
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   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.
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   6. A monitor nozzle.
      [Webster 1913 Suppl.]

   Monitor top, the raised central portion, or clearstory, of
      a car roof, having low windows along its sides.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Varanus niloticus
    n 1: destroys crocodile eggs [syn: African monitor, Varanus
         niloticus]