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]