dilute solution in which the solute may be regarded as obeying Henry's law, so that all the solute activity coefficients may be approximated to 1.
Source:
Physical Chemistry Division, unpublished
See also:
PAC, 1994, 66, 533. 'Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)' on page 545 (https://doi.org/10.1351/pac199466030533)