An agreement between the EU and external countries concerning issues of Community and member state competence - for example the Cotonou Agreement. Such agreements must be signed by both the EU and its member states.
The Lisbon Treaty has established the Union as a legal personality (Art. 47 TEU). It opened for mixed agreements on foreign policy and community topics.
Hence, in areas where the Union legislates, the EU can sign various kinds of agreements on behalf of the member states, who are consequently not allowed to negotiate international agreements on their own in these areas.
The so-called 'competence of external negotiation', has become a sole competence of the EU - member states are no longer allowed to negotiate within this area on their own.
See also Agreements with other Countries.