mussel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various marine bivalve mollusks that attach to hard surfaces in intertidal areas with byssal threads, especially the edible members of the family Mytilidae and in particular Mytilus edulis, a blue-black species of the North Atlantic Ocean, raised commercially for food.
• noun Any of numerous freshwater bivalve mollusks of the order Unionoida that burrow in the sand or mud of rivers, streams, and ponds.
• noun Any of several similar bivalve mollusks, such as the zebra mussel.

from The Century Dictionary.

• noun Any one of many bivalve mollusks of various genera and species.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) Any one of many species of marine bivalve shells of the genus Mytilus, and related genera, of the family Mytidæ. The common mussel (Mytilus edulis; see Illust. under byssus), and the larger, or horse, mussel (Modiola modiolus), inhabiting the shores both of Europe and America, are edible. The former is extensively used as food in Europe.
• noun (Zoöl.) Any one of numerous species of Unio, and related fresh-water genera; -- called also river mussel. See Naiad, and Unio.
• noun (Zoöl.) the grayback whale. See Gray whale, under Gray.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small edible bivalve shellfish of the families Unionidae (fresh water mussels) and Mytilidae (salt water mussels).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun black marine bivalves usually steamed in wine
• noun marine or freshwater bivalve mollusk that lives attached to rocks etc.