The ratio of the quantity of the solvent to that of the other phase.
Unless otherwise specified the phase ratio refers to the phase volume ratio.
If other aspects of the phase ratio are employed viz. phase mass ratio, phase flow ratio, these should be specified.
PAC, 1993, 65, 2373
(Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993))
on page 2385
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook