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
    n 1: an arc of colored light in the sky caused by refraction of
         the sun's rays by rain
    2: an illusory hope; "chasing rainbows"

Moby Thesaurus II by Grady Ward, 1.0:

185 Moby Thesaurus words for "rainbow":
   Amytal, Amytal pill, Dalmatian, Demerol, Dolophine, H, Luminal,
   Luminal pill, M, Mickey Finn, Nembutal, Nembutal pill, Seconal,
   Seconal pill, Tuinal, Tuinal pill, alcohol, amobarbital sodium,
   analgesic, angry clouds, anodyne, anthelion, antigorite, antisun,
   aura, aureole, barb, barbiturate, barbiturate pill, black cat,
   black clouds, black stuff, blue, blue angel, blue devil,
   blue heaven, blue velvet, broken mirror, butterfly, calmative,
   candy cane, chameleon, cheetah, chimera, chloral hydrate,
   chromatic, chrysotile, circle, codeine, codeine cough syrup, cold,
   colorific, coloring, confetti, cool, corona, countersun,
   crazy quilt, depressant, depressor, dichromatic, dolly, downer,
   dream, fantasy, firedog, gathering clouds, glory, glowing,
   goofball, halcyon bird, halo, hard stuff, harlequin, harmonious,
   heroin, hop, horse, hypnotic, illusion, iridescence, iris, jaguar,
   junk, knockout drops, laudanum, leopard, liquor, lotus,
   lunar corona, lunar halo, mackerel, mackerel sky, many-colored,
   marble, marbled paper, matching, medley, meperidine, methadone,
   mock moon, mock sun, moire, monochromatic, monochrome, monochromic,
   moon dog, morphia, morphine, mother-of-pearl,
   mother-of-pearl cloud, motley, nacre, nacreousness, narcotic,
   nimbus, ocelot, opal, opalescence, ophite, opiate, opium, owl,
   pacifier, pain killer, paraselene, paregoric, parhelic circle,
   parhelion, parti-colored, patchwork quilt, peacock, pearliness,
   pen yan, phenobarbital, phenobarbital sodium, pigmentary, pipe,
   polychromatic, prismatic, purple heart, quietener, raven, red,
   ring, scag, secobarbital sodium, sedative, serpentine,
   serpentine marble, shit, shooting star, shot silk, sleep-inducer,
   sleeper, sleeping draught, sleeping pill, smack, sodium thiopental,
   solar corona, solar halo, somnifacient, soother, soothing syrup,
   soporific, spectral, spectrum, storm clouds, stormy petrel,
   sun dog, tar, thundercloud, thunderhead, tinctorial, tingent,
   toning, tortoise shell, tranquilizer, turps, variegated, warm,
   white stuff, yellow, yellow jacket, zebra



Easton's 1897 Bible Dictionary:

Rainbow
   caused by the reflection and refraction of the rays of the sun
   shining on falling rain. It was appointed as a witness of the
   divine faithfulness (Gen. 9:12-17). It existed indeed before,
   but it was then constituted as a sign of the covenant. Others,
   however (as Delitzsch, Commentary on Pentateuch), think that it
   "appeared then for the first time in the vault and clouds of
   heaven." It is argued by those holding this opinion that the
   atmosphere was differently constituted before the Flood. It is
   referred to three other times in Scripture (Ezek. 1:27, 28; Rev.
   4:1-3; 10:1).


U.S. Gazetteer Places (2000):

Rainbow, CA -- U.S. Census Designated Place in California
   Population (2000):    2026
   Housing Units (2000): 780
   Land area (2000):     16.074997 sq. miles (41.634049 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    16.074997 sq. miles (41.634049 sq. km)
   FIPS code:            59248
   Located within:       California (CA), FIPS 06
   Location:             33.411213 N, 117.149209 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Rainbow, CA
    Rainbow