The Collaborative International Dictionary of English v.0.48:

Killifish \Kil"li*fish`\, n. (Zool.)
   Any one of several small American cyprinodont fishes of the
   genus Fundulus and allied genera. They live equally well in
   fresh and brackish water, or even in the sea. They are
   usually striped or barred with black. Called also minnow,
   and brook fish. See Minnow.
   [1913 Webster]

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]

WordNet (r) 3.0 (2006):

killifish
    n 1: small mostly marine warm-water carp-like schooling fishes;
         used as bait or aquarium fishes or in mosquito control