any substance capable of reducing the speed of a given reaction.
Origin of retardant
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a substance that reduces the rate of a chemical reaction
having a slowing effect
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
1550s, "retardation," from Middle French retardance, from retarder (see retard (v.)). In reference to resistance to fire, 1948.
Online Etymology Dictionary, © 2010 Douglas Harper