In national politics (or low politics), regionalisation is a process of dividing a political entity — typically a country — into smaller regions, and transferring power from the central government to the regions. Opposite process is called unitarisation.
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'.
Regionalisation and unitarisation are often confused with, respectively, decentralisation and centralisation.
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