The other party that participates in a financial transaction. Every transaction must have a counterparty in order for the transaction to go through. More specifically, every buyer of an asset must be paired up with a seller that is willing to sell and vice versa.

All trades require some sort of counterparty. For example, the counterparty to the option buyer would be the option writer. One of the risks involved in any transaction is counterparty risk, which is the risk that the counterparty will be unable to fulfill his or her duties. However, in many financial transactions the counterparty is unknown.