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.
   [1913 Webster]

   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.
         [1913 Webster]

   Lunar rainbow, a fainter arch or rainbow, formed by the
      moon.

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rainbow trout
    n 1: flesh of Pacific trout that migrate from salt to fresh
         water
    2: found in Pacific coastal waters and streams from lower
       California to Alaska [syn: rainbow trout, Salmo
       gairdneri]