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