'A' value

Also contains definition of: Winstein–Holness A value

The conformational preference of an equatorial compared to an axial substituent in a monosubstituted cyclohexane. This steric substituent parameter equals Δ r G 0 in kcal mol −1 for the equatorial to axial equilibration on cyclohexane. The values are also known as 'Winstein–Holness' A values.
Source:
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1087