The active component(s) primarily responsible for transfer of a solute from one phase to the other.
The term extracting agent is a synonym but solvent and ligand should not be used in this context.
Certain extractants that consist of liquids immiscible with water (e.g. tributyl phosphate or certain ketones) might comprise the only component of the initial organic phase but extractant(s) can also be dissolved in diluent.
PAC, 1993, 65, 2373
(Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993))
on page 2380
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