transition species
Any intermediate species in an elementary reaction. An activated complex is an example of a transition species, but the latter term also includes other intermediate species. Transition species have also been called transition states, but there is then danger of confusion with a true activated complex, which is also commonly known as a transition state.
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 190 (