rain·bow ˈrānˌbō/Submit noun noun: rainbow; plural noun: rainbows an arch of colors formed in the sky in certain circumstances, caused by the refraction and dispersion of the sun's light by rain or other water droplets in the atmosphere. any display of the colors of the spectrum produced by dispersion of light. a wide range or variety of related and typically colorful things. "a rainbow of medals decorated his chest" many-colored. modifier noun: rainbow "a big rainbow packet of felt pens" short for rainbow trout.