In International Relations regionalisation (or regionalism) is actually the set of processes that lead states to work together in an international system on a regional scale. From the realist perspective of International Relations it is actually an outward expansion from the nation state.
Behind the process of regionalisation lies the concept of regionalism. This can be seen as the normative aspects, or values, that underly regionalisation e.g. the (contested) European identity. However, it is also a theoretical tool for analysis of international relations. For example the concept of security regime in regional security would not be possible without the analytic tool of 'regionalism'.
The E.U. Model
C.S.N. (Comunidad de Sudamericana Naciones)
The African Union