a. Formed with or like a rainbow. [ 1913 Webster ]

n. [ AS. regenboga, akin to G. regenbogen. See Rain, and Bow anything bent. ] A bow or arch exhibiting, in concentric bands, the several colors of the spectrum, and formed in the part of the hemisphere opposite to the sun by the refraction and reflection of the sun's rays in drops of falling rain. [ 1913 Webster ]

☞ Besides the ordinary bow, called also primary rainbow, which is formed by two refractions and one reflection, there is also another often seen exterior to it, called the secondary rainbow, concentric with the first, and separated from it by a small interval. It is formed by two refractions and two reflections, is much fainter than the primary bow, and has its colors arranged in the reverse order from those of the latter. [ 1913 Webster ]

Lunar rainbow, a fainter arch or rainbow, formed by the moon. --
Marine rainbow,
Sea bow, a similar bow seen in the spray of waves at sea. --
Rainbow trout (Zool.), a bright-colored trout (Salmo irideus), native of the mountains of California, but now extensively introduced into the Eastern States, Japan, and other countries; -- called also brook trout, mountain trout, and golden trout. --
Rainbow wrasse. (Zool.) See under Wrasse. --
Supernumerary rainbow, a smaller bow, usually of red and green colors only, sometimes seen within the primary or without the secondary rainbow, and in contact with them. [ 1913 Webster ]