Definition of "mussel"

Any of several groups of bivalve shellfish with elongated, asymmetrical shells.

1. A freshwater mussel, usually edible, of the order Unionida in subclass Palaeoheterodonta.  examples

2. A saltwater mussel, usually edible, of the order Mytilida in subclass Pteriomorphia.  examples

3. Any of certain other bivalves of somewhat similar appearance, such as the zebra mussel and quagga mussel of the family Dreissenidae in subclass Heterodonta.  examples