Co-ownership is a system whereby two football clubs own the contract of a player jointly, although the player is only registered to play for one club. It is not a universal system, but is used in some countries, including Argentina, Chile and Uruguay. It was formerly commonplace in Italy, though the practice has now been abolished there.
This type of deal differs from third-party ownership, in that in the latter, the player's contract is owned by a non-footballing entity, such as a management company.