Real Dictionary

rainbow
countable noun: A rainbow is an arch of different colours that you can sometimes see in the sky when it is raining.

rainbow in American English
an arc or ring containing the colors of the spectrum in consecutive bands, formed in the sky by the refraction, reflection, and dispersion of light in rain or fog
of many colors
noun: a bow or arc of prismatic colors appearing in the heavens opposite the sun and caused by the refraction and reflection of the sun's rays in drops of rain

rainbow in British English
noun: a bow-shaped display in the sky of the colours of the spectrum, caused by the refraction and reflection of the sun's rays through rain or mist
noun: a member of the Rainbow Guides, the youngest group of girls (aged 5-7 years) in The Guide Association