symbols: $x_{{\rm{v}},{\ce{A}}}$, $w_{{\rm{v}},{\ce{A}}}$
also defines: solvent mass fraction
https://doi.org/10.1351/goldbook.15361
Mole or mass fraction of a solvent in a solution containing \(s\) solute constituents (\(i = 1, 2, \ldots, s\)) and \(p\) solvent constituents (\(i = s + 1, s + 2, \ldots, p\)). For a solvent \(\ce{A}\), \[x_{{\rm{v}},{\ce{A}}} = x_{\ce{A}}/\sum \limits_{i=s+1}^{p} x_{i}\] where \[\sum \limits_{i=1}^{p} x_{i} = \sum \limits_{i=1}^{s} x_{i} + \sum \limits_{i=s+1}^{p} x_{i} = 1\] and analogously for \(w_{{\rm{v}},{\ce{A}}}\), where the first and last summations are over all solvent constituents.
See also: Jänecke coordinates