A prefix to specify configuration (absolute or relative) or conformation. For example
R,
S;
r,
s;
P,
M;
Re,
Si;
E,
Z;
ap,
sp; etc.
Source:
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2218 (https://doi.org/10.1351/pac199668122193)