The Collaborative International Dictionary of English v.0.48:

Opah \O"pah\, n. (Zool.)
   A large oceanic fish (Lampris guttatus), inhabiting the
   Atlantic Ocean. It is remarkable for its brilliant colors,
   which are red, green, and blue, with tints of purple and
   gold, covered with round silvery spots. Called also king of
   the herrings.
   [1913 Webster]

WordNet (r) 3.0 (2006):

opah
    n 1: large elliptical brightly colored deep-sea fish of Atlantic
         and Pacific and Mediterranean [syn: opah, moonfish,
         Lampris regius]