The Collaborative International Dictionary of English v.0.48:

Crucian carp \Cru"cian carp`\ (-shan k?rp`). [Cf. Sw. karussa,
   G. karausche, F. carousse, -assin, corassin, LL. coracinus,
   Gr. ????? a sort of fish.] (Zool.)
   A kind of European carp (Carasius vulgaris), inferior to
   the common carp; -- called also German carp.
   [1913 Webster]

   Note: The gibel or Prussian carp is now generally considered
         a variety of the crucian carp, or perhaps a hybrid
         between it and the common carp.
         [1913 Webster]

WordNet (r) 3.0 (2006):

crucian carp
    n 1: European carp closely resembling wild goldfish [syn:
         crucian carp, Carassius carassius, Carassius
         vulgaris]