A multicoloured arch in the sky, produced by prismatic refraction of light within droplets of rain in the air.

Any prismatic refraction of light showing a spectrum of colours.

(often used with “of”) A wide assortment; a varied multitude.

a rainbow of possibilities

An illusion, mirage.

Many electoral promises are rainbows, vanishing soon after poll day.

A curveball, particularly a slow one.

In Texas hold 'em or Omaha hold 'em, a flop that contains three different suits.

Rainbow trout.

To brighten with, or as with, a rainbow; to pattern with the colours of the rainbow.

To take the appearance of a rainbow.

Multicolored.

Made up of several races or ethnicities, or (more broadly) of several cultural or ideological factions.

LGBT.

(chiefly of a flop) Composed entirely of different suits.