EU analysts talk about the "widening" and "deepening" of the EU.
Deepening involves, among other things, the introduction of qualified majority rather than unanimous voting for making supranational European law. This entails further integration. Widening involves enlargement by covering either more member states or more policy areas.
Up to now EU deepening has always been a condition for EU widening, particularly with regard to the EU treaties. This is mainly because the founding countries want a federal development in Europe that will cover all the existing members.
The possibility of using the 'enhanced cooperation' provisions of the Treaties - introduced by the Nice Treaty - to permit some member states to integrate more closely among themselves, envisages a departure from this approach, but these provisions have not so far been used.
The Lisbon Treaty also provides for enhanced cooperation decided by qualified majority and makes possible structured cooperation for some member states in defence.