mid-14c., accioun, "cause or grounds for a lawsuit," from Anglo-French accioun, Old French accion, action (12c.) "action; lawsuit, case," from Latin actionem (nominative actio) "a putting in motion; a performing, a doing; public acts, official conduct; lawsuit, legal action" (source also of Spanish accion, Italian azione), noun of action from past-participle stem of agere "to do" (from PIE root *ag- "to drive, draw out or forth, move").
Spelling with the restored Latin -t- begins in 15c. Meaning "active exertion, activity" is from late 14c. Sense of "something done, an act, deed" is late 14c. Meaning "military fighting" is from 1590s. Meaning "way in which (a firearm, etc.) acts" is from 1845. As a film director's command, it is attested from 1923.
The meaning "noteworthy or important activity" in a modern sense by 1933, as in the figurative phrase a piece of the action (by 1965), perhaps from a sense of action in card-playing jargon by 1914.
No "action" can be had on a bet until the card bet upon appears. If it does not appear after a turn has been made, the player is at liberty to change his bet, or to remove it altogether. Each bet is made for the turn only, unless the player chooses to leave it until he gets some action on it. [from "Faro" in "Hoyle's Games," A.L. Burt Company, New York: 1914]
But there are uses of action as far back as c. 1600 that seem to mean "noteworthy activity." The meaning "excitement" is recorded from 1968. In action "in a condition of effective operation" is from 1650s. Phrase actions speak louder than words is attested from 1731. Action-packed is attested from 1953, originally of movies.
"action in resistance or response to another action or power," 1640s, from re- "back, again, anew" + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), a noun of action formed in Late Latin from the past-participle stem of Latin reagere "react," from re- "back" + agere "to do, perform."
Originally a word in physics and dynamics. In chemistry, "mutual or reciprocal action of chemical agents upon each other," by 1836. The general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.
1831 in calculus; 1855 as "action of differentiating, condition of being different," noun of action from differentiate. As "action of noting a difference," 1866.
"act of subjecting to the action of chlorine," 1854, noun of action from chlorinate (v.).
also magnetisation, "action of magnetizing; condition of being magnetized," 1801, noun of action from magnetize.