peripheral atom

in organic reaction mechanisms
https://doi.org/10.1351/goldbook.P04497
A secondary @R05227-1@ (non-core) @R05227-2@ which at some point in the mechanistic @ST06775@ is situated in the molecule containing the @C01341@.
Source:
PAC, 1989, 61, 23. (System for symbolic representation of reaction mechanisms (Recommendations 1988)) on page 27 [Terms] [Paper]