catalyst (n.)youdaoicibaDictYouDict
"substance which speeds a chemical reaction but itself remains unchanged," 1902, formed in English (on analogy of analyst) from catalysis. Figurative use by 1943.
reagent (n.)youdaoicibaDictYouDict
1785, from re- + agent "substance that produces a chemical reaction."
stoichiometry (n.)youdaoicibaDictYouDict
"science of calculating the quantities of chemical elements involved in chemical reactions," 1807, from German Stöchiometrie (1792), coined by German chemist Jeremias Benjamin Richter (1762-1807) from Greek stoikheion "one of a row; shadow-line of a sundial," in plural "the elements" (from PIE *steigh- "to stride, step, rise") + -metry. Related: Stoichiometric.
actinicyoudaoicibaDictYouDict
"(Of light or lighting) able to cause photochemical reactions, as in photography, through having a significant short wavelength or ultraviolet component", Mid 19th century: from Greek aktis, aktin- 'ray' + -ic.