What is Interventionism?

Legal Definition
Interventionism is a term for a policy of non-defensive (proactive) activity undertaken by a nation-state, or other geo-political jurisdiction of a lesser or greater nature, to manipulate an economy and/or society. The most common applications of the term are for economic interventionism (a state's intervention in its own economy), and foreign interventionism (a state's intervention in the affairs of another nation as part of its foreign policy).
-- Wikipedia