The logarithm of the ratio of the total concentration (neutral and charged species) of a compound in a nonpolar phase, traditionally water-saturated octan-1-ol, to that in water at a given
\(\pu{pH}\).
Source:
PAC, 2016, 88, 239. 'Glossary of terms used in computational drug design, part II (IUPAC Recommendations 2015)' on page 249 (https://doi.org/10.1515/pac-2012-1204)