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.
   [1913 Webster]

         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.
         [1913 Webster]

   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.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

RAIN WATER. The water which naturally falls from the clouds.
     2. No one has a right to build his house so as to cause the rain water
to fall over his neighbor's land; 1 Rolle's Ab. 107; 2 Leo. 94; 1 Str. 643;
Fortesc. 212; Bac. Ab. Action on. the case, F.; 5 Co. 101; 2 Rolle, Ab. 565,
1. 10; 1 Com. Dig. Action upon the case for a nuisance, A; unless he has
acquired a right by a grant or prescription.
     3. When the land remains in a state of nature, says a learned writer,
and by the natural descent, the rain water would descend from the superior
estate over the lower, the latter is necessarily subject to receive such
water. 1 Lois des Batimens, 15, 16. Vide 2 Roll. 140; Dig. 39, 3; 2 Bouv.
Inst. n. 1608.