Etymology
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reaction (n.)

"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.

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reactionary (adj.)

1831, "of or pertaining to political reaction, tending to revert from a more to a less advanced policy," on model of French réactionnaire (19c.), from réaction (see reaction). In Marxist use by 1858 as "tending toward reversing existing tendencies," opposed to revolutionary and used opprobriously in reference to opponents of communism. Non-political use, "of or pertaining to a (chemical, etc.) reaction" (1847) is rare. As a noun, "person considered reactionary," especially in politics, one who seeks to check or undo political action, by 1855.

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react (v.)

1640s, "to exert, as a thing acted upon, an opposite action upon the agent," from re- "back" + act (v.). Related: Reacted; reacting (1610s). For sense development, see reaction. The verb meaning "to perform again, do a second time" (often written re-actand given full pronunciation of the prefix to distinguish it from react) is from 1650s, from the "again" sense in re-.

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reactive (adj.)

1712, "repercussive, echoing," a sense now obsolete, from react + -ive. By 1822 as "caused by a reaction;" 1888 as "susceptible to (chemical) reaction." Related: Reactively; reactiveness; reactivity.

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anaphylaxis (n.)
"severe allergic reaction," 1905, from Latin anaphylaxis, perhaps based on French anaphylaxie (1902); see anaphylactic.
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double-take (n.)

"exaggerated reaction to surprise," 1922, from double (adj.) + take (n.). Originally in stage comedy acting.

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Intifada (n.)
"Palestinian revolt," 1985, from Arabic, literally "a jumping up" (in reaction to something), from the verb intafada "to be shaken, shake oneself."
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straight-faced (adj.)
1938, of persons, "with visage showing no emotion or reaction," from expression keep a straight face (1897), from straight (adj.).
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sis-boom-bah 
cheerleading chant, originally (1867) an echoic phrase imitating the sound of a skyrocket flight (sis), the burst of the fireworks (boom), and the reaction of the crowd ((b)ah).
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catalyst (n.)

"substance which speeds a chemical reaction but itself remains unchanged," 1900, formed in English (on analogy of analyst) from catalysis. Figurative use by 1943.

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