ideal mixture

A mixture of substances B, C, ... is called an ideal mixture when $a B = x B$ , $a C = x C$ , ... or $f B = 1$ , $f C = 1$ where $a B$ and $f B$ are the relative activity and the activity coefficient, respectively, of a substance B in a liquid or solid mixture.

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 543