rainbow

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An arc of spectral colors, usually identified as red, orange, yellow, green, blue, indigo, and violet, that appears in the sky opposite the sun as a result of the refractive dispersion of sunlight in drops of rain or mist.
2. n. A similar arc or band, as one produced by a prism or by iridescence.
3. n. A graded display of colors.
4. n. An illusory hope: chasing the rainbow of overnight success.
5. n. A diverse assortment or collection.

Century Dictionary and Cyclopedia

1. n. A bow, or an arc of a circle, consisting of the prismatic colors, formed by the refraction and reflection of rays of light from drops of rain or vapor, appearing in the part of the heavens opposite to the sun. When large and strongly illuminated, the rainbow presents the appearance of two concentric arches, the inner being called the primary and the outer the secondary rainbow. Each is formed of the colors of the solar spectrum, but the colors are arranged in reversed order, the red forming the exterior ring of the primary bow and the interior of the secondary. The primary bow is formed by rays of the sun that enter the upper part of falling drops of rain, and undergo two refractions and one reflection; the secondary, by rays that enter the under part of rain drops, and undergo two refractions and two reflections. Hence, the colors of the secondary bow are fainter than those of the primary. The rainbow is regarded as a symbol of divine beneficence toward man, from its being made the token of the covenant that the earth should never again be destroyed by a flood (Gen. ix. 13–17). Smaller bows, sometimes circular and very brilliant, are often seen through masses of mist or spray, as from a waterfall or from waves about a ship. (See fog-bow.) The moon sometimes forms a bow or arch of light, more faint than that formed by the sun, and called a lunar rainbow.
2. n. In heraldry, the representation of a half-ring divided into seven concentric narrow rings and arched upward, each end resting on a clump of clouds. To avoid the difficulty of finding seven different tinctures, the number of concentric rings is sometimes diminished to three, usually azure, or, and g ules—that is, blue, gold, and red.
3. n. In ornithology, a humming-bird of tho genus Diphlogena, containing two most brilliantly plumaged species, D. iris of Bolivia, and D. hesperus of Ecuador.
4. n. The rainbow-fish.
5. To color like a rainbow, or so brightly as to suggest the rainbow; curve over or surround, as with a rainbow.

Wiktionary

1. n. A multicoloured arch in the sky, produced by prismatic refraction of light within droplets of rain in the air.
2. n. Any prismatic refraction of light showing a spectrum of colours.
3. n. A wide assortment; a varied multitude.
4. n. An illusion, mirage
5. n. A curveball, particularly a slow one
6. n. In Texas hold 'em or Omaha hold 'em, a flop that contains three different suits
7. adj. Multicoloured.
8. adj. this redundant sense?) Having, or shining with, the colors of the rainbow: iridescent.
9. adj. Composed entirely of different suits.
10. adj. LGBT.
11. v. To pattern with many colours, like a rainbow.

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. an arc of colored light in the sky caused by refraction of the sun's rays by rain
2. n. an illusory hope