The Collaborative International Dictionary of English v.0.48:

Lobster \Lob"ster\, n. [AS. loppestre, lopystre prob., corrupted
   fr. L. locusta a marine shellfish, a kind of lobster, a
   locust. Cf. Locust.] (Zool.)
   1. Any large macrurous crustacean used as food, esp. those of
      the genus Homarus; as the American lobster (Homarus
      Americanus), and the European lobster (Homarus
      vulgaris). The Norwegian lobster (Nephrops Norvegicus)
      is similar in form. All these have a pair of large unequal
      claws. The spiny lobsters of more southern waters,
      belonging to Palinurus, Panulirus, and allied genera,
      have no large claws. The fresh-water crayfishes are
      sometimes called lobsters.
      [1913 Webster]

   2. As a term of opprobrium or contempt: A gullible, awkward,
      bungling, or undesirable person. [Slang]
      [Webster 1913 Suppl.]

   Lobster caterpillar (Zool.), the caterpillar of a European
      bombycid moth (Stauropus fagi); -- so called from its
      form.

   Lobster louse (Zool.), a copepod crustacean (Nicotho["e]
      astaci) parasitic on the gills of the European lobster.
      [1913 Webster]

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):

Homarus americanus
    n 1: lobster of Atlantic coast of America [syn: American
         lobster, Northern lobster, Maine lobster, Homarus
         americanus]