A rainbow is an optical and meteorological phenomenon caused by precipitation in an atmosphere having a prismatic effect upon white light (typically from a sun), and creating the appearance of an arc of color through the sky.
In 2370, Gideon Seyetik painted the following mental picture of Da Vinci Falls for Benjamin Sisko: "Imagine...Water cascading off cliffs the height of Mount Everest and thundering through continuous rainbows, straight into an ocean the color of sapphire wine." (DS9: "Second Sight")
In Irish legend, a leprechaun's pot of gold was to be found at the end of a rainbow's arch. In 2376, when Fair Haven inhabitant Seamus asked Tom Paris how to find the pot of gold in Glen Abbey, Paris told him to follow the rainbow. (VOY: "Spirit Folk")