ACT is generally used as a short form of Activity. Activity is a state of being active in behavior, action or any particular type of tasks which need alertness and energy.
For example: If you are running, it is activity. If you are dancing, it is activity. If you are thinking, it is a activity. Everything you do as a work for a particular purpose is a activity. Some activities are done for the pleasure i.e. playing cricket is an activity which requires two teams and a group of people.