The Collaborative International Dictionary of English v.0.48:

Trout \Trout\ (trout), n. [AS. truht, L. tructa, tructus; akin
   to Gr. trw`kths a sea fish with sharp teeth, fr. trw`gein to
   gnaw.]
   1. (Zool.) Any one of numerous species of fishes belonging to
      Salmo, Salvelinus, and allied genera of the family
      Salmonidae. They are highly esteemed as game fishes and
      for the quality of their flesh. All the species breed in
      fresh water, but after spawning many of them descend to
      the sea if they have an opportunity.
      [1913 Webster]
      [1913 Webster]

   Note: The most important European species are the river, or
         brown, trout (Salmo fario), the salmon trout, and the
         sewen. The most important American species are the
         brook, speckled, or red-spotted, trout (Salvelinus
         fontinalis) of the Northern United States and Canada;
         the red-spotted trout, or Dolly Varden (see Malma);
         the lake trout (see Namaycush); the black-spotted,
         mountain, or silver, trout (Salmo purpuratus); the
         golden, or rainbow, trout (see under Rainbow); the
         blueback trout (see Oquassa); and the salmon trout
         (see under Salmon.) The European trout has been
         introduced into America.
         [1913 Webster]

   2. (Zool.) Any one of several species of marine fishes more
      or less resembling a trout in appearance or habits, but
      not belonging to the same family, especially the
      California rock trouts, the common squeteague, and the
      southern, or spotted, squeteague; -- called also
      salt-water trout, sea trout, shad trout, and gray
      trout. See Squeteague, and Rock trout under Rock.
      [1913 Webster]

   Trout perch (Zool.), a small fresh-water American fish
      (Percopsis guttatus), allied to the trout, but
      resembling a perch in its scales and mouth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

trout
    n 1: flesh of any of several primarily freshwater game and food
         fishes
    2: any of various game and food fishes of cool fresh waters
       mostly smaller than typical salmons