Pronunciation: (ak'tu-viz"um),[key] —n. 1. the doctrine or practice of vigorous action or involvement as a means of achieving political or other goals, sometimes by demonstrations, protests, etc.
2. Philos. 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.