The Collaborative International Dictionary of English v.0.48:

Plaice \Plaice\, n. [F. plaise, plais, prob. fr. L. platessa
   flatish, plaice. See Place.] (Zool.)
   (a) A European food fish (Pleuronectes platessa), allied to
       the flounder, and growing to the weight of eight or ten
       pounds or more.
   (b) A large American flounder (Paralichthys dentatus;
       called also brail, puckermouth, and summer
       flounder. The name is sometimes applied to other allied
       species. [Written also plaise.]
       [1913 Webster]

   Plaice mouth, a mouth like that of a plaice; a small or wry
      mouth. [R.] --B. Jonson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plaice
    n 1: flesh of large European flatfish
    2: large European food fish [syn: plaice, Pleuronectes
       platessa]