The prefix for an elementary reaction or some part thereof indicating that the bond(s) undergoing primitive changes form part of a ring in the transition state of the elementary reaction. This prefix is used for nonpericyclic processes only and is replaced by 'cyclo' for pericyclic processes.
Source:
PAC, 1989, 61, 23. 'System for symbolic representation of reaction mechanisms (Recommendations 1988)' on page 34 (https://doi.org/10.1351/pac198961010023)