In professional football, a transfer is the action taken whenever a player under contract moves between clubs. It refers to the transferring of a player's registration from one association football club to another. In general, the players can only be transferred during a transfer window and according to the rules set by a governing body. A negotiated transfer fee is agreed financial compensation paid from an interested club to the club that possesses the player's exclusive contracted playing rights. When a player moves from one club to another, their old contract is terminated whilst the player and their new destination club will both negotiate on new contract terms. As such, the transfer fee functions as financial compensation for the early mutually agreed termination of the contract of a professional football player. Transfer fees are contingent on the player’s current football abilities, future potential, duration of the existing contract, amount of future salary owed and the willingness of clubs to agree on an economic equilibrium through supply and demand.