partial specific volume, vB 

https://doi.org/10.1351/goldbook.P04422
Change in volume of a system when a small amount of component is added divided by the mass of the added component. The systematic name is partial massic volume.
Source:
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 986 [Terms] [Paper]