WordNet sense 1 (the state of being active):

activeness, activity

WordNet sense 2 (a process existing in or produced by nature (rather than by the intent of human beings)):

natural action, natural process

WordNet sense 3 (the operating part that transmits power to a mechanism):

action mechanism

WordNet sense 4 (institute legal proceedings against; file a suit against):

litigate, process, sue, process

WordNet sense 5 (put in effect):

accomplish, execute, fulfil, fulfill, carry through, carry out

WordNet sense 6 (a judicial proceeding brought by one party against another; one party prosecutes another for a wrong done or for protection of a right or for prevention of a wrong):

action at law, legal action

WordNet sense 7 (a military engagement):

military action