early 15c., "one who feels," agent noun from feel (v.). Of animal organs, 1660s. Transferred sense of "proposal put forth to observe the reaction it gets" is from 1830. Related: Feelers.
"cause (something) to happen," 1930, an image from trigger (n.). In recent use especially psychological, "to cause an intense and usually negative emotional reaction (in a person or animal)," by 1986. Related: Triggered; triggering.
mid-15c. as "a rejoinder, a reply" (a sense now archaic or obsolete); 1520s, "the return or bounding back of something after striking, act of flying back on collision with another body" in reference to a ball, from rebound (v.); rebounding in this sense is from late 14c.
In modern sports, from 1917 in ice hockey, 1920 in basketball. Transferred and figurative senses from 1560s; the meaning "period of reaction or renewed activity after disturbance" is from 1570s, hence "during a period of reaction after the end of a romantic or marital relationship" (1859).
"cause or accelerate (a reaction) by acting as a catalyst; cause to begin," 1871, probably a back-formation from catalysis on model of analyze/analysis. Related: Catalyzed; catalyzing. Probably influenced by French catalyser (1842).
"of or pertaining to a severe allergic reaction," 1905, with -ic + medical Latin noun anaphylaxis "exaggerated susceptibility," from Greek ana- "up" (see ana-) + phylaxis "protection," from phylax "guardian, watcher, protector," a word of unknown origin. Compare prophylactic. Anaphylactic shock is attested by 1916.
1640s, "one who or that which checks or delays," agent noun from retard (v.). Scientific sense of "substance which slows down a reaction" by 1878. Specifically of braking mechanisms by 1937, originally on railroad cars.