Thanks for the query.
It is Onion. (Latin 'unio').
Origin of word:
Latin: unio -> Old French: Oignon -> English: Onion
Definition of unio as per Merriam - Webster Dictionary: 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.
Source: The web
unio at Merriam - Webster: http://www.merriam-webster.com/dictionary/unio