The Collaborative International Dictionary of English v.0.48:

Refraction \Re*frac"tion\ (r?*fr?k"sh?n), n. [F. r['e]fraction.]
   1. The act of refracting, or the state of being refracted.
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   2. The change in the direction of ray of light, heat, or the
      like, when it enters obliquely a medium of a different
      density from that through which it has previously moved.
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            Refraction out of the rarer medium into the denser,
            is made towards the perpendicular.    --Sir I.
                                                  Newton.
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   3. (Astron.)
      (a) The change in the direction of a ray of light, and,
          consequently, in the apparent position of a heavenly
          body from which it emanates, arising from its passage
          through the earth's atmosphere; -- hence distinguished
          as atmospheric refraction, or astronomical refraction.
      (b) The correction which is to be deducted from the
          apparent altitude of a heavenly body on account of
          atmospheric refraction, in order to obtain the true
          altitude.
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   Angle of refraction (Opt.), the angle which a refracted ray
      makes with the perpendicular to the surface separating the
      two media traversed by the ray.

   Conical refraction (Opt.), the refraction of a ray of light
      into an infinite number of rays, forming a hollow cone.
      This occurs when a ray of light is passed through crystals
      of some substances, under certain circumstances. Conical
      refraction is of two kinds; external conical refraction,
      in which the ray issues from the crystal in the form of a
      cone, the vertex of which is at the point of emergence;
      and internal conical refraction, in which the ray is
      changed into the form of a cone on entering the crystal,
      from which it issues in the form of a hollow cylinder.
      This singular phenomenon was first discovered by Sir W. R.
      Hamilton by mathematical reasoning alone, unaided by
      experiment.

   Differential refraction (Astron.), the change of the
      apparent place of one object relative to a second object
      near it, due to refraction; also, the correction required
      to be made to the observed relative places of the two
      bodies.

   Double refraction (Opt.), the refraction of light in two
      directions, which produces two distinct images. The power
      of double refraction is possessed by all crystals except
      those of the isometric system. A uniaxial crystal is said
      to be optically positive (like quartz), or optically
      negative (like calcite), or to have positive, or negative,
      double refraction, according as the optic axis is the axis
      of least or greatest elasticity for light; a biaxial
      crystal is similarly designated when the same relation
      holds for the acute bisectrix.

   Index of refraction. See under Index.

   Refraction circle (Opt.), an instrument provided with a
      graduated circle for the measurement of refraction.

   Refraction of latitude, longitude, declination, right
   ascension, etc., the change in the apparent latitude,
      longitude, etc., of a heavenly body, due to the effect of
      atmospheric refraction.

   Terrestrial refraction, the change in the apparent altitude
      of a distant point on or near the earth's surface, as the
      top of a mountain, arising from the passage of light from
      it to the eye through atmospheric strata of varying
      density.
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The Collaborative International Dictionary of English v.0.48:

Double \Dou"ble\ (d[u^]b"'l), a. [OE. doble, duble, double, OF.
   doble, duble, double, F. double, fr. L. duplus, fr. the root
   of duo two, and perh. that of plenus full; akin to Gr.
   diplo`os double. See Two, and Full, and cf. Diploma,
   Duple.]
   1. Twofold; multiplied by two; increased by its equivalent;
      made twice as large or as much, etc.
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            Let a double portion of thy spirit be upon me. -- 2
                                                  Kings ii. 9.
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            Darkness and tempest make a double night. --Dryden.
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   2. Being in pairs; presenting two of a kind, or two in a set
      together; coupled.
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            [Let] The swan, on still St. Mary's lake,
            Float double, swan and shadow.        --Wordsworth.
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   3. Divided into two; acting two parts, one openly and the
      other secretly; equivocal; deceitful; insincere.
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            With a double heart do they speak.    -- Ps. xii. 2.
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   4. (Bot.) Having the petals in a flower considerably
      increased beyond the natural number, usually as the result
      of cultivation and the expense of the stamens, or stamens
      and pistils. The white water lily and some other plants
      have their blossoms naturally double.
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   Note: Double is often used as the first part of a compound
         word, generally denoting two ways, or twice the number,
         quantity, force, etc., twofold, or having two.
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   Double base, or Double bass (Mus.), the largest and
      lowest-toned instrument in the violin form; the
      contrabasso or violone.

   Double convex. See under Convex.

   Double counterpoint (Mus.), that species of counterpoint or
      composition, in which two of the parts may be inverted, by
      setting one of them an octave higher or lower.

   Double court (Lawn Tennis), a court laid out for four
      players, two on each side.

   Double dagger (Print.), a reference mark ([dag]) next to
      the dagger ([dagger]) in order; a diesis.

   Double drum (Mus.), a large drum that is beaten at both
      ends.

   Double eagle, a gold coin of the United States having the
      value of 20 dollars.

   Double entry. See under Bookkeeping.

   Double floor (Arch.), a floor in which binding joists
      support flooring joists above and ceiling joists below.
      See Illust. of Double-framed floor.

   Double flower. See Double, a., 4.

   Double-framed floor (Arch.), a double floor having girders
      into which the binding joists are framed.

   Double fugue (Mus.), a fugue on two subjects.

   Double letter.
      (a) (Print.) Two letters on one shank; a ligature.
      (b) A mail requiring double postage.

   Double note (Mus.), a note of double the length of the
      semibreve; a breve. See Breve.

   Double octave (Mus.), an interval composed of two octaves,
      or fifteen notes, in diatonic progression; a fifteenth.

   Double pica. See under Pica.

   Double play (Baseball), a play by which two players are put
      out at the same time.

   Double plea (Law), a plea alleging several matters in
      answer to the declaration, where either of such matters
      alone would be a sufficient bar to the action. --Stephen.

   Double point (Geom.), a point of a curve at which two
      branches cross each other. Conjugate or isolated points of
      a curve are called double points, since they possess most
      of the properties of double points (see Conjugate). They
      are also called acnodes, and those points where the
      branches of the curve really cross are called crunodes.
      The extremity of a cusp is also a double point.

   Double quarrel. (Eccl. Law) See Duplex querela, under
      Duplex.

   Double refraction. (Opt.) See Refraction.

   Double salt. (Chem.)
      (a) A mixed salt of any polybasic acid which has been
          saturated by different bases or basic radicals, as the
          double carbonate of sodium and potassium,
          NaKCO3.6H2O.
      (b) A molecular combination of two distinct salts, as
          common alum, which consists of the sulphate of
          aluminium, and the sulphate of potassium or ammonium.


   Double shuffle, a low, noisy dance.

   Double standard (Polit. Econ.), a double standard of
      monetary values; i. e., a gold standard and a silver
      standard, both of which are made legal tender.

   Double star (Astron.), two stars so near to each other as
      to be seen separate only by means of a telescope. Such
      stars may be only optically near to each other, or may be
      physically connected so that they revolve round their
      common center of gravity, and in the latter case are
      called also binary stars.

   Double time (Mil.). Same as Double-quick.

   Double window, a window having two sets of glazed sashes
      with an air space between them.
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WordNet (r) 3.0 (2006):

double refraction
    n 1: splitting a ray into two parallel rays polarized
         perpendicularly [syn: double refraction, birefringence]