The Collaborative International Dictionary of English v.0.48:

Minnow \Min"now\, n. [OE. menow, cf. AS. myne; also OE. menuse,
   OF. menuise small fish; akin to E. minish, minute.] [Written
   also minow.]
   [1913 Webster]
   1. (Zool.) A small European fresh-water cyprinoid fish
      (Phoxinus laevis, formerly Leuciscus phoxinus);
      sometimes applied also to the young of larger kinds; --
      called also minim and minny. The name is also applied
      to several allied American species, of the genera
      Phoxinus, Notropis, or Minnilus, and Rhinichthys.
      [1913 Webster]

   2. (Zool.) Any of numerous small American cyprinodont fishes
      of the genus Fundulus, and related genera. They live
      both in fresh and in salt water. Called also killifish,
      minny, and mummichog.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mummichog \Mum"mi*chog\, n. [Amer. Indian name.] (Zool.)
   Any one of several species of small American cyprinodont
   fishes of the genus Fundulus, and of allied genera; the
   killifishes; -- called also minnow. [Written also
   mummychog, mummachog.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

mummichog
    n 1: silver-and-black killifish of saltwater marshes along the
         Atlantic coast of the United States [syn: mummichog,
         Fundulus heteroclitus]