An Etymological Dictionary of Astronomy and Astrophysics

1. The doctrine or practice of vigorous action or involvement as a means of achieving political or other goals, sometimes by demonstrations, protests, etc. (Dictionary.com). See also → activist.
   

2. Philosophy: a) A theory that the essence of reality is pure activity, especially 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 (Dictionary.com).

See also: → active; → action; → -ism.