The Collaborative International Dictionary of English v.0.48:

Chub \Chub\, n. [This word seems to signify a large or thick
   fish. Cf. Sw. kubb a short and thick piece of wood, and perh.
   F. chabot chub.] (Zool.)
   A species to fresh-water fish of the Cyprinid[ae] or Carp
   family. The common European species is Leuciscus cephalus;
   the cheven. In America the name is applied to various fishes
   of the same family, of the genera Semotilus, Squalius,
   Ceratichthys, etc., and locally to several very different
   fishes, as the tautog, black bass, etc.
   [1913 Webster]

   Chub mackerel (Zool.), a species of mackerel (Scomber
      colias) in some years found in abundance on the Atlantic
      coast, but absent in others; -- called also bull
      mackerel, thimble-eye, and big-eye mackerel.

   Chub sucker (Zool.), a fresh-water fish of the United
      States (Erimyzon sucetta); -- called also creekfish.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Leuciscus cephalus
    n 1: European freshwater game fish with a thick spindle-shaped
         body [syn: chub, Leuciscus cephalus]