Meaning

noun (chemistry) a process in which one or more substances are changed into others

noun an idea evoked by some experience

noun a bodily process occurring due to the effect of some antecedent stimulus or agent

originInherited from Middle English reaccion, from Old French reaction, from Latin reāctiō, from the verb reagō, from re- (“again”) + agō (“to act”); more at re-, action; equivalent to react + -ion.

Etymology from Wiktionary, CC BY-SA 3.0