The Collaborative International Dictionary of English v.0.48:

maine lobster \maine lobster\ n.
   1. Any of several cold-water lobsters with large tender
      claws, caught from Maine to the Carolinas.

   Syn: American lobster, Northern lobster.
        [WordNet 1.5]

   2. A lobster (Homarus americanus) found off the Atlantic
      coast of North America.

   Syn: American lobster, Northern lobster, Homarus
        americanus.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Maine lobster
    n 1: flesh of cold-water lobsters having large tender claws;
         caught from Maine to the Carolinas [syn: American
         lobster, Northern lobster, Maine lobster]
    2: lobster of Atlantic coast of America [syn: American
       lobster, Northern lobster, Maine lobster, Homarus
       americanus]