rainbow |ˈrānˌbō| noun 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. • many-colored: a big rainbow packet of felt pens. PHRASES at the end of the rainbow used to refer to something much sought after but impossible to attain.