Refers to a small group of countries willing and able to enter into closer co-operation with one another.
In 1994 the German Christian Democrats launched the idea of a "hard core" group of EU members (based around the founding EU countries Germany, France, Italy and Benelux) to promote closer European integration. The "Karl Lamers paper" issued on that occasion initiated what is now known as enhanced co-operation.
In February 2003 a French MEP, Jean Louis Bourlanges, proposed a hard core based around a federation established by France and Germany.