CFD. A contract between two people that mirrors the situation of trading a security, without actually buying or selling the security. The two parties make a contract that the seller will pay the buyer the difference in price after a certain period of time if the designated security's price increases, and the buyer will in return pay the seller the difference in price if the security's price decreases. CFDs are traded in over the counter markets in many countries, although they are not allowed in the United States.