Subsidiarity

General
The general aim of the principle of subsidiarity is to guarantee a degree of independence for a lower authority in relation to a higher body or for a local authority in relation to central government. It therefore involves the sharing of powers between several levels of authority, a principle which forms the institutional basis for federal states.

European Union
When applied in the context of the European Union, the principle of subsidiarity serves to regulate the exercise of the Union's non-exclusive powers. It rules out Union intervention when an issue can be dealt with effectively by Member States at central, regional or local level and means that the Union is justified in exercising its powers when Member States are unable to achieve the objectives of a proposed action satisfactorily and added value can be provided if the action is carried out at Union level.

Source

 * European Parliament, Fact Sheets on the European Union (full-text).