Mercantile means concerned with trade: buying and selling goods, often between countries or in a city that grew rich that way. A mercantile nation lives on trade; mercantile law is the law that merchants use. You will see it in history, economics, and legal writing. Though actually, in the street people almost never say it. They say trade, business, or commercial. Mercantile feels stiff and a little old — the textbook word when a writer wants to sound serious. It describes a kind of activity or world, not a person. The person is a merchant.