The Collaborative International Dictionary of English v.0.48:
Rainbow \Rain"bow`\ (r[=a]n"b[=o]`), n. [AS. regenboga, akin to
G. regenbogen. See Rain, and Bow anything bent.]
A bow or arch exhibiting, in concentric bands, the several
colors of the spectrum, and formed in the part of the
hemisphere opposite to the sun by the refraction and
reflection of the sun's rays in drops of falling rain.
Note: Besides the ordinary bow, called also primary rainbow,
which is formed by two refractions and one reflection,
there is also another often seen exterior to it, called
the secondary rainbow, concentric with the first, and
separated from it by a small interval. It is formed by
two refractions and two reflections, is much fainter
than the primary bow, and has its colors arranged in
the reverse order from those of the latter.
Lunar rainbow, a fainter arch or rainbow, formed by the
Marine rainbow, Sea bow, a similar bow seen in the spray
of waves at sea.
Rainbow trout (Zool.), a bright-colored trout (Salmo
irideus), native of the mountains of California, but now
extensively introduced into the Eastern States, Japan, and
other countries; -- called also brook trout, mountain
trout, and golden trout.
Rainbow wrasse. (Zool.) See under Wrasse.
Supernumerary rainbow, a smaller bow, usually of red and
green colors only, sometimes seen within the primary or
without the secondary rainbow, and in contact with them.