third body
A species, other than the reactant itself, which brings about the energization of a molecule that can undergo a @M03989@ process, or brings about a combination reaction between atoms or radicals. In the latter case a third body is also called a @C00973@.
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 189 [Terms] [Paper]