1) Arc seen in waterfalls 2) Water-light phenomenon 
Similar to a rainbow but appears in a clear sky, without rain and does not arc to earth. Also used of a rainbow produced when sunlight shines through spray. the first impression is that of an upside-down rainbow The circumzenithal arc or circumzenith arc (CZA), also called the Bravais` arc, is an optical phenomenon similar in appearance to a rain....
• (n.) A rainbow; an iris.
Sun'bow` noun A rainbow; an iris. Byron. 
a bow or arc of prismatic colors like a rainbow, appearing in the spray of cataracts, waterfalls, fountains, etc.