From AMS Glossary
A rainbow formed by a light source reflected by an extended water surface; not to be confused with a rainbow seen reflected in a still body of water.
When the water is calm, the center of a reflection rainbow is at the same elevation as the sun but in the opposite part of the sky. Such a bow intersects the ordinary rainbow (primary or secondary) at the horizon. When the water is rough (which it often is under the windy conditions in the vicinity of a shower) a family of reflection bows is formed, the envelope of which produces an essentially vertical bow, which intersects the ordinary bow at the horizon.
Humphreys, W. J. 1940. Physics of the Air. 3d ed., . 498–499.