retarder

https://doi.org/10.1351/goldbook.R05355
This term, used mainly in polymerization kinetics, refers to a substance that reduces the rate of reaction. Synonymous with inhibitor.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 185 (https://doi.org/10.1351/pac199668010149)