Trade allows the building of caravans, a special unit for establishing trade routes between cities and for helping building wonders.
It is a prerequisite of banking and medicine.
Civilopedia entry[]
One of the oldest and most widespread social institutions is the exchange of goods, or Trade. At the most basic level of Trade, two people exchange items with each other. Ideally, the items given are in surplus and the items received are desired. The result is that both are better off, having exchanged extras for something wanted. People, as well as nations, normally have a comparative advantage over others in the production of some good or service. Through Trade, this advantage benefits both parties. Trade has also proven very important in the exchange of ideas.