As nouns the difference between tendency and action
is that
tendency is a likelihood of behaving in a particular way or going in a particular direction; a tending toward while
action is something done so as to accomplish a purpose.
As an interjection action is
demanding or signifying the start of something, usually an act or scene of a theatric performance.
As a verb action is
(management) to act on a request etc, in order to put it into effect.