in organic reaction mechanisms
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.
PAC, 1989, 61, 23. 'System for symbolic representation of reaction mechanisms (Recommendations 1988)' on page 34 (