A participant atom which is neither core nor peripheral and which carries other groups to and from the molecule containing the core atom(s)
but which does not itself become covalently attached to that molecule.
PAC, 1989, 61, 23
(System for symbolic representation of reaction mechanisms (Recommendations 1988))
on page 27
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook