diluent

in solvent extraction
also contains definition of: cosolvent
https://doi.org/10.1351/goldbook.D01735
The liquid or homogeneous mixture of liquids in which extractant(s) and possible modifier(s) may be dissolved to form the solvent phase.
Notes:
  1. The term carrier, which implies an inert diluent is not recommended.
  2. Although the diluent may well be a single liquid or even the major portion of the extracting phase, the term solvent should not be used in this sense as it has a much wider meaning in the context of liquid-liquid extraction, although the term cosolvent may be used in certain circumstances.
  3. The diluent by itself does not extract the main (extractable) solute appreciably.
Source:
PAC, 1993, 65, 2373. (Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)) on page 2380 [Terms] [Paper]