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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   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):

monitor
    n 1: someone who supervises (an examination) [syn: proctor,
         monitor]
    2: someone who gives a warning so that a mistake can be avoided
       [syn: admonisher, monitor, reminder]
    3: an ironclad vessel built by Federal forces to do battle with
       the Merrimac
    4: display produced by a device that takes signals and displays
       them on a television screen or a computer monitor [syn:
       monitor, monitoring device]
    5: electronic equipment that is used to check the quality or
       content of electronic transmissions
    6: a piece of electronic equipment that keeps track of the
       operation of a system continuously and warns of trouble
    7: any of various large tropical carnivorous lizards of Africa
       and Asia and Australia; fabled to warn of crocodiles [syn:
       monitor, monitor lizard, varan]
    v 1: keep tabs on; keep an eye on; keep under surveillance; "we
         are monitoring the air quality"; "the police monitor the
         suspect's moves" [syn: monitor, supervise]
    2: check, track, or observe by means of a receiver

Moby Thesaurus II by Grady Ward, 1.0:

179 Moby Thesaurus words for "monitor":
   ARRL, Big Brother, CRT, Cassandra, Dutch uncle, Jeremiah, Polonius,
   TV man, admonisher, adviser, amateur radio operator, announcer,
   annunciator, apprentice teacher, audit, auditor, authority,
   autodidact, autopsy, backseat driver, boatswain, boss, buttinsky,
   cameraman, canvass, cautioner, channel, check, check out,
   check over, check up on, chief, communicant, communicator,
   comptroller, confidant, consultant, control engineer, controller,
   counsel, counselor, custodian, display, educatee, enlightener,
   examine, expert, expert witness, flagman, floor manager, floorman,
   floorwalker, follow, foreman, gaffer, ganger, give an examination,
   go over, gossipmonger, graduate assistant, grapevine, guard,
   guardian, guide, ham, head, headman, informant, information center,
   information medium, informer, inquirer, inspect, inspector,
   instructor, intern, interviewee, keep track of, kibitzer, learner,
   lighthouse keeper, look at, look over, lookout, lookout man,
   meddler, mentor, mixer, mouthpiece, nestor, newsmonger,
   noncommissioned officer, notifier, observe, orienter, overhaul,
   overlook, overman, oversee, overseer, paraprofessional, pass over,
   pass under review, peer at, peruse, pore over, postmortem,
   practice teacher, praepostor, preceptist, prefect, press, proctor,
   prophet of doom, public relations officer, publisher, pupil,
   pupil teacher, radio, radio electrician, radio engineer,
   radio operator, radio technician, radioman, radiotelegrapher,
   radiotrician, record, reporter, review, run over, scan, scholar,
   scrutinize, self-taught person, sentinel, sentry,
   set an examination, signaler, signalman, sirdar, size, size up,
   slave driver, source, spokesman, straw boss, student,
   student assistant, studier, study, subforeman, super, superintend,
   superintendent, supervise, supervisor, survey, surveyor,
   take stock of, take the measure, taskmaster, teacher,
   teaching assistant, teaching fellow, teaching intern, television,
   television technician, teller, tipster, tout, trace, track,
   trainee, vet, visitor, warner, watch, watchdog, witness



The Free On-line Dictionary of Computing (30 December 2018):

monitor

   1. A cathode-ray tube and associated electronics connected
   to a computer's video output.  A monitor may be either
   monochrome (black and white) or colour (RGB).  Colour
   monitors may show either digital colour (each of the red,
   green and blue signals may be either on or off, giving eight
   possible colours: black, white, red, green, blue, cyan,
   magenta and yellow) or analog colour (red, green and blue
   signals are continuously variable allowing any combination to
   be displayed).  Digital monitors are sometimes known as TTL
   because the voltages on the red, green and blue inputs are
   compatible with TTL logic chips.

   See also gamut, multisync, visual display unit.

   2. A programming language construct which encapsulates
   variables, access procedures and initialisation code within an
   abstract data type.  The monitor's variable may only be
   accessed via its access procedures and only one process may be
   actively accessing the monitor at any one time.  The access
   procedures are critical sections.  A monitor may have a
   queue of processes which are waiting to access it.

   3. A hardware device that measures electrical events such as
   pulses or voltage levels in a digital computer.

   4. To oversee a program during execution.  For example, the
   monitor function in the Unix C library enables profiling
   of a certain range of code addresses.  A histogram is produced
   showing how often the program counter was found to be at
   each position and how often each profiled function was called.

   Unix man page: monitor(3).

   5. A control program within the operating system that
   manages the allocation of system resources to active
   programs.

   6. A program that measures software performance.