Solution in which the sum of amount fractions of all the solutes is small compared to 1.
Source:
PAC, 1994, 66, 533. 'Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)' on page 535 (https://doi.org/10.1351/pac199466030533)
See also:
Purple Book, 1st ed., p. 57 (http://old.iupac.org/publications/books/author/metanomski.html)