• (a.) Acting of or by one`s self or by itself; -- said especially of a machine or mechanism which is made to perform of or for itself what is usually done by human agency; automatic; as, a self-acting feed apparatus; a self-acting mule; a self-acting press.
Self`-act'ing adjective Acting of or by one's self or by itself; -- said especially of a machine or mechanism which is made to perform of or for itself what is usually done by human agency; automatic; as, a self-acting feed apparatus; a self-acting mule; a self-acting press. 
[adj] - designed to activate or move or regulate itself
self-activating adjective designed to activate or move or regulate itself; `a self-activating sprinkler system`