The Collaborative International Dictionary of English v.0.48:

Gauge \Gauge\, n. [Written also gage.]
   1. A measure; a standard of measure; an instrument to
      determine dimensions, distance, or capacity; a standard.
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            This plate must be a gauge to file your worm and
            groove to equal breadth by.           --Moxon.
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            There is not in our hands any fixed gauge of minds.
                                                  --I. Taylor.
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   2. Measure; dimensions; estimate.
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            The gauge and dimensions of misery, depression, and
            contempt.                             --Burke.
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   3. (Mach. & Manuf.) Any instrument for ascertaining or
      regulating the dimensions or forms of things; a templet or
      template; as, a button maker's gauge.
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   4. (Physics) Any instrument or apparatus for measuring the
      state of a phenomenon, or for ascertaining its numerical
      elements at any moment; -- usually applied to some
      particular instrument; as, a rain gauge; a steam gauge.
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   5. (Naut.)
      (a) Relative positions of two or more vessels with
          reference to the wind; as, a vessel has the weather
          gauge of another when on the windward side of it, and
          the lee gauge when on the lee side of it.
      (b) The depth to which a vessel sinks in the water.
          --Totten.
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   6. The distance between the rails of a railway.
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   Note: The standard gauge of railroads in most countries is
         four feet, eight and one half inches. Wide, or broad,
         gauge, in the United States, is six feet; in England,
         seven feet, and generally any gauge exceeding standard
         gauge. Any gauge less than standard gauge is now called
         narrow gauge. It varies from two feet to three feet six
         inches.
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   7. (Plastering) The quantity of plaster of Paris used with
      common plaster to accelerate its setting.
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   8. (Building) That part of a shingle, slate, or tile, which
      is exposed to the weather, when laid; also, one course of
      such shingles, slates, or tiles.
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   Gauge of a carriage, car, etc., the distance between the
      wheels; -- ordinarily called the track.

   Gauge cock, a stop cock used as a try cock for ascertaining
      the height of the water level in a steam boiler.

   Gauge concussion (Railroads), the jar caused by a car-wheel
      flange striking the edge of the rail.

   Gauge glass, a glass tube for a water gauge.

   Gauge lathe, an automatic lathe for turning a round object
      having an irregular profile, as a baluster or chair round,
      to a templet or gauge.

   Gauge point, the diameter of a cylinder whose altitude is
      one inch, and contents equal to that of a unit of a given
      measure; -- a term used in gauging casks, etc.

   Gauge rod, a graduated rod, for measuring the capacity of
      barrels, casks, etc.

   Gauge saw, a handsaw, with a gauge to regulate the depth of
      cut. --Knight.

   Gauge stuff, a stiff and compact plaster, used in making
      cornices, moldings, etc., by means of a templet.

   Gauge wheel, a wheel at the forward end of a plow beam, to
      determine the depth of the furrow.

   Joiner's gauge, an instrument used to strike a line
      parallel to the straight side of a board, etc.

   Printer's gauge, an instrument to regulate the length of
      the page.

   Rain gauge, an instrument for measuring the quantity of
      rain at any given place.

   Salt gauge, or Brine gauge, an instrument or contrivance
      for indicating the degree of saltness of water from its
      specific gravity, as in the boilers of ocean steamers.

   Sea gauge, an instrument for finding the depth of the sea.


   Siphon gauge, a glass siphon tube, partly filled with
      mercury, -- used to indicate pressure, as of steam, or the
      degree of rarefaction produced in the receiver of an air
      pump or other vacuum; a manometer.

   Sliding gauge. (Mach.)
      (a) A templet or pattern for gauging the commonly accepted
          dimensions or shape of certain parts in general use,
          as screws, railway-car axles, etc.
      (b) A gauge used only for testing other similar gauges,
          and preserved as a reference, to detect wear of the
          working gauges.
      (c) (Railroads) See Note under Gauge, n., 5.

   Star gauge (Ordnance), an instrument for measuring the
      diameter of the bore of a cannon at any point of its
      length.

   Steam gauge, an instrument for measuring the pressure of
      steam, as in a boiler.

   Tide gauge, an instrument for determining the height of the
      tides.

   Vacuum gauge, a species of barometer for determining the
      relative elasticities of the vapor in the condenser of a
      steam engine and the air.

   Water gauge.
      (a) A contrivance for indicating the height of a water
          surface, as in a steam boiler; as by a gauge cock or
          glass.
      (b) The height of the water in the boiler.

   Wind gauge, an instrument for measuring the force of the
      wind on any given surface; an anemometer.

   Wire gauge, a gauge for determining the diameter of wire or
      the thickness of sheet metal; also, a standard of size.
      See under Wire.
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The Collaborative International Dictionary of English v.0.48:

Rain \Rain\ (r[=a]n), n. [OE. rein, AS. regen; akin to OFries.
   rein, D. & G. regen, OS. & OHG. regan, Icel., Dan., & Sw.
   regn, Goth. rign, and prob. to L. rigare to water, to wet;
   cf. Gr. bre`chein to wet, to rain.]
   Water falling in drops from the clouds; the descent of water
   from the clouds in drops.
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         Rain is water by the heat of the sun divided into very
         small parts ascending in the air, till, encountering
         the cold, it be condensed into clouds, and descends in
         drops.                                   --Ray.
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         Fair days have oft contracted wind and rain. --Milton.
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   Note: Rain is distinguished from mist by the size of the
         drops, which are distinctly visible. When water falls
         in very small drops or particles, it is called mist;
         and fog is composed of particles so fine as to be not
         only individually indistinguishable, but to float or be
         suspended in the air. See Fog, and Mist.
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   Rain band (Meteorol.), a dark band in the yellow portion of
      the solar spectrum near the sodium line, caused by the
      presence of watery vapor in the atmosphere, and hence
      sometimes used in weather predictions.

   Rain bird (Zool.), the yaffle, or green woodpecker. [Prov.
      Eng.] The name is also applied to various other birds, as
      to Saurothera vetula of the West Indies.

   Rain fowl (Zool.), the channel-bill cuckoo (Scythrops
      Novae-Hollandiae) of Australia.

   Rain gauge, an instrument of various forms for measuring
      the quantity of rain that falls at any given place in a
      given time; a pluviometer; an ombrometer.

   Rain goose (Zool.), the red-throated diver, or loon. [Prov.
      Eng.]

   Rain prints (Geol.), markings on the surfaces of stratified
      rocks, presenting an appearance similar to those made by
      rain on mud and sand, and believed to have been so
      produced.

   Rain quail. (Zool.) See Quail, n., 1.

   Rain water, water that has fallen from the clouds in rain.
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WordNet (r) 3.0 (2006):

rain gauge
    n 1: gauge consisting of an instrument to measure the quantity
         of precipitation [syn: rain gauge, rain gage,
         pluviometer, udometer]