Volume, or volumetric, Strain, or dilatation (the relative variation of the volume) is the trace of the tensor $\vartheta$.
Applicable units are those of quantitykind:DimensionlessRatio
http://en.wikipedia.org/wiki/Deformation_(mechanics)
$\vartheta = \frac{\Delta V}{V_0}$, where $\Delta V$ is the increase in volume and $V_0$ is the volume in a reference state to be specified.
http://www.iso.org/iso/catalogue_detail?csnumber=31889