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Synonyms for Action (same or very similar meaning)

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

From the ODE community, based on WordNetadd/edit