A person who shares a partnership with one or more others; a joint partner in a business or endeavor.
From co- + partner (from Middle English, meaning a sharer, from part). Partner itself comes from Old French parçonaire, one who shares portions.
Copartner is one of the few 'co-' words that's actually gaining ground in modern business English—it's being revived to describe equals in startup culture, replacing the older hierarchical term 'partner.'
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