Definition of unio
1 capitalized : the type genus of the family Unionidae comprising freshwater mussels that have an oblong shell pearly within and covered without by a greenish or blackish epidermis
2 plural unios \-ōz\ or unionides : any mussel of Unio or a related genus
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Origin and Etymology of unio
New Latin, from Latin, a large pearl — more at 3union
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