The Collaborative International Dictionary of English v.0.48:

Dab \Dab\, n. [Perh. so named from its quickness in diving
   beneath the sand. Cf. Dabchick.] (Zool.)
   A name given to several species of flounders, esp. to the
   European species, Pleuronectes limanda. The American rough
   dab is Hippoglossoides platessoides.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Hippoglossoides platessoides
    n 1: large American food fish [syn: American plaice,
         Hippoglossoides platessoides]