"Hard core" refers to a limited group of countries able and willing to develop 'closer cooperation'.
This concept found practical expression in the implementation of the Schengen area, which enabled a group of Member States to progressively curtail border checks and to establish freedom of movement for persons. This hard core was initially set up outside the EU's institutional framework, but was eventually integrated into the treaties through its incorporation into the Amsterdam Treaty.
The Amsterdam Treaty formally adopted the use of the hard core concept through its introduction of the 'closer cooperation' mechanism. 'Closer cooperation' enables a certain number of Member States to further the construction of Europe within the EU's institutional framework.