Dual coincidence of wants
A dual coincidence of wants occurs when two economic agents meet and find that agent A possesses what agent B wants, while agent B possess what agent A wants. Thus, they are able to trade the items to satisfy each other's need.
In a barter economy, dual coincidence of wants is necessary for any exchange to occur. Money's function as a medium of exchange eliminates dual conincidence of wants as a precondition for trade.