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.
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
Fair days have oft contracted wind and rain. --Milton.
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.
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.
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
Rain quail. (Zool.) See Quail, n., 1.
Rain water, water that has fallen from the clouds in rain.