Synonymous with catalyst.
See: catalyst (in solvent extraction), kinetic synergist, modifier (in solvent extraction)
Source:
PAC, 1993, 65, 2373. 'Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)' on page 2380 (https://doi.org/10.1351/pac199365112373)