In economics, the double coincidence of wants is a situation that must occur in order for barter to be possible. It is a major reason that barter is a very awkward basis for an economic system.
In order for barter to occur, two people's wants must coincide. Let us say that I have some wheat that I want to trade. Let us further say that you have a chicken I would like to have. In order for this trade to occur, our wants must coincide. I must want your chicken. In addition, you must want my wheat. If either of these conditions does not apply (for example, I want your chicken, but you want tomatoes, not wheat) we cannot trade.
The double coincidence of wants, then, exists when both parties to a barter exchange have what the other wants.