the doctrine or practice of vigorous action or involvement as a means of achieving political or other goals, sometimes by demonstrations, protests, etc.
2.
Philosophy.
a.
a theory that the essence of reality is pure activity, esp. spiritual activity, or process.
b.
a theory that the relationship between the mind and the objects of perception depends upon the action of the mind.
ac·tiv·ism (āk'tə-vĭz'əm) n. The use of direct, often confrontational action, such as a demonstration or strike, in opposition to or support of a cause. ac·tiv·ist'ic adj.