Merriam-Webster's Collegiate® Dictionary, 11th Edition
RAINBOW
I. noun Date: before 12th century an arc or circle that exhibits in concentric bands the colors of the spectrum and that is formed opposite the sun by the refraction and reflection of the sun's rays in raindrops, spray, or mist,
2. a multicolored array, a wide assortment or range , an illusory goal or hope, rainbow trout , rainbowlike adjective
II. adjective
Date: 1652
having many colors, of, relating to, or being people of different races or cultural backgrounds