From Neo-Latin reagens 'acting back,' from Latin re- + agere 'to act,' for a substance that responds to another in a chemical reaction.
A substance used in a chemical reaction to detect, measure, or produce other substances.
From Neo-Latin reagentem, from re- 'back, again' + agentem 'acting,' present participle of agere 'to do, drive.' A reagent is literally 'that which acts back'—it responds to or reacts with another substance. The term emerged from 18th-century chemistry as the discipline systematized its vocabulary. Key roots: *h₂eǵ- (Proto-Indo-European: "to drive,