The present article analyses the issues of the legal definition of the European Union. It has been noticed a while ago that the EU has outgrown the “gown” of a typical international organisation and that it has acquired some features specific to a State-like entity. It is no coincidence that some authors accentuate that the EU is a specific, unconventional international organisation, some – that it is an entity, very similar to a State (an incomplete federation, a post-modern State), others, (...) not being satisfied with the traditional distinction between an international organisation and a State, are trying to develop the third category, calling the EU sui generis organisation or supranational organisation. The arguments concerning the EU model of integration continue even after the Treaty of Lisbon entered into force. Not being able to fit this legal phenomenon into the usual typological scheme we need to consider the development of a new theory enabling to define the new phenomenon. The definition of this kind of category meets several obstacles: it is a unique legal phenomenon, it is constantly changing and, furthermore, the hybrid nature of the Union induces to qualify the EU following either the criteria of international or of national constitutional law while elaborating the abovementioned synthetic category. (shrink)