A substance introduced into a reaction system in order to bring about an initiation reaction.
Sources:
PAC, 1976, 46, 71. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II: Heterogeneous Catalysis' on page 74 (https://doi.org/10.1351/pac197646010071)
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 170 (https://doi.org/10.1351/pac199668010149)