The Collaborative International Dictionary of English v.0.48:

Level \Lev"el\ (l[e^]v"[e^]l), n. [OE. level, livel, OF. livel,
   F. niveau, fr. L. libella level, water level, a plumb level,
   dim. of libra pound, measure for liquids, balance, water
   poise, level. Cf. Librate, Libella.]
   1. A line or surface to which, at every point, a vertical or
      plumb line is perpendicular; a line or surface which is
      everywhere parallel to the surface of still water; -- this
      is the true level, and is a curve or surface in which all
      points are equally distant from the center of the earth,
      or rather would be so if the earth were an exact sphere.
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   2. A horizontal line or plane; that is, a straight line or a
      plane which is tangent to a true level at a given point
      and hence parallel to the horizon at that point; -- this
      is the apparent level at the given point.
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   3. An approximately horizontal line or surface at a certain
      degree of altitude, or distance from the center of the
      earth; as, to climb from the level of the coast to the
      level of the plateau and then descend to the level of the
      valley or of the sea.
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            After draining of the level in Northamptonshire.
                                                  --Sir M. Hale.
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            Shot from the deadly level of a gun.  --Shak.
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   4. Hence, figuratively, a certain position, rank, standard,
      degree, quality, character, etc., conceived of as in one
      of several planes of different elevation.
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            Providence, for the most part, sets us on a level.
                                                  --Addison.
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            Somebody there of his own level.      --Swift.
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            Be the fair level of thy actions laid
            As temperance wills and prudence may persuade.
                                                  --Prior.
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   5. A uniform or average height; a normal plane or altitude; a
      condition conformable to natural law or which will secure
      a level surface; as, moving fluids seek a level.
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            When merit shall find its level.      --F. W.
                                                  Robertson.
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   6. (Mech. & Surv.)
      (a) An instrument by which to find a horizontal line, or
          adjust something with reference to a horizontal line.
      (b) A measurement of the difference of altitude of two
          points, by means of a level; as, to take a level.
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   7. A horizontal passage, drift, or adit, in a mine.
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   Air level, a spirit level. See Spirit level (below).

   Box level, a spirit level in which a glass-covered box is
      used instead of a tube.

   Carpenter's level, Mason's level, either the plumb level
      or a straight bar of wood, in which is imbedded a small
      spirit level.

   Level of the sea, the imaginary level from which heights
      and depths are calculated, taken at a mean distance
      between high and low water.

   Line of levels, a connected series of measurements, by
      means of a level, along a given line, as of a railroad, to
      ascertain the profile of the ground.

   Plumb level, one in which a horizontal bar is placed in
      true position by means of a plumb line, to which it is at
      right angles.

   Spirit level, one in which the adjustment to the horizon is
      shown by the position of a bubble in alcohol or ether
      contained in a nearly horizontal glass tube, or a circular
      box with a glass cover.

   Surveyor's level, a telescope, with a spirit level
      attached, and with suitable screws, etc., for accurate
      adjustment, the whole mounted on a tripod, for use in
      leveling; -- called also leveling instrument.

   Water level, an instrument to show the level by means of
      the surface of water in a trough, or in upright tubes
      connected by a pipe.
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The Collaborative International Dictionary of English v.0.48:

Water level \Wa"ter lev"el\
   1. The level formed by the surface of still water.
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   2. A kind of leveling instrument. See under Level, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

water level
    n 1: the level of the surface of a body of water
    2: underground surface below which the ground is wholly
       saturated with water; "spring rains had raised the water
       table" [syn: water table, water level, groundwater
       level]
    3: a line corresponding to the surface of the water when the
       vessel is afloat on an even keel; often painted on the hull
       of a ship [syn: waterline, water line, water level]
    4: a water gauge that shows the level by showing the surface of
       the water in a trough or U-shaped tube