The Collaborative International Dictionary of English v.0.48:

Plumb \Plumb\ (pl[u^]m), n. [F. plomb, L. plumbum lead, a leaden
   ball or bullet; cf. Gr. mo`lybos, mo`libos, mo`lybdos. Cf.
   Plummet, Plunge.]
   A little mass or weight of lead, or the like, attached to a
   line, and used by builders, etc., to indicate a vertical
   direction; a plummet; a plumb bob. See Plumb line, below.
   [1913 Webster]

   Plumb bob. See Bob, 4.

   Plumb joint, in sheet-metal work, a lap joint, fastened by
      solder.

   Plumb level. See under Level.

   Plumb line.
   (a) The cord by which a plumb bob is suspended; a plummet.
   (b) A line directed to the center of gravity of the earth.

   Plumb rule, a narrow board with a plumb line, used by
      builders and carpenters.
      [1913 Webster]

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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

            After draining of the level in Northamptonshire.
                                                  --Sir M. Hale.
      [1913 Webster]

            Shot from the deadly level of a gun.  --Shak.
      [1913 Webster]

   4. Hence, figuratively, a certain position, rank, standard,
      degree, quality, character, etc., conceived of as in one
      of several planes of different elevation.
      [1913 Webster]

            Providence, for the most part, sets us on a level.
                                                  --Addison.
      [1913 Webster]

            Somebody there of his own level.      --Swift.
      [1913 Webster]

            Be the fair level of thy actions laid
            As temperance wills and prudence may persuade.
                                                  --Prior.
      [1913 Webster]

   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.
      [1913 Webster]

            When merit shall find its level.      --F. W.
                                                  Robertson.
      [1913 Webster]

   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.
          [1913 Webster]

   7. A horizontal passage, drift, or adit, in a mine.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plumb level
    n 1: a carpenter's level with a plumb line at right angles to it