The Collaborative International Dictionary of English v.0.48:

Haddock \Had"dock\ (-d[u^]k), n. [OE. hadok, haddok, of unknown
   origin; cf. Ir. codog, Gael. adag, F. hadot.] (Zool.)
   A marine food fish (Melanogrammus [ae]glefinus), allied to
   the cod, inhabiting the northern coasts of Europe and
   America. It has a dark lateral line and a black spot on each
   side of the body, just back of the gills. Galled also
   haddie, and dickie.
   [1913 Webster]

   Norway haddock, a marine edible fish (Sebastes marinus)
      of Northern Europe and America. See Rose fish.
      [1913 Webster]

WordNet (r) 3.0 (2006):

haddock
    n 1: lean white flesh of fish similar to but smaller than cod;
         usually baked or poached or as fillets sauteed or fried
    2: important food fish on both sides of the Atlantic; related to
       cod but usually smaller [syn: haddock, Melanogrammus
       aeglefinus]