Coupling is the interdependence among individual components influenced by the interfaces, the kind of connections, and the kind of communications.
There are many different kinds of coupling. In data coupling, two components share the same data. Examples of data coupling include the usage of a data base to connect two systems. In stamp coupling, two components share a common data structure type which is passed in parameter calls. In common-environment coupling, two components communicate through a third party: the common-environment. In external coupling two components share a common-global data item.