What is the term for the change in volume per unit volume?

The correct term for the change in volume per unit volume is "volumetric strain." Volumetric strain refers specifically to the measure of deformation representing the volume change of a material in response to an applied stress. It is expressed as the ratio of the change in volume to the original volume of the body, which effectively quantifies how much a material expands or contracts under pressure.

Dilatation, while often related to volume changes, typically refers to an increase in volume or the action of expanding, making it a broader term that does not specifically capture the idea of volume change relative to original volume as volumetric strain does. Shear strain and tensile strain deal with different types of deformation. Shear strain measures the change in shape of an object (change in angles) under shear stress, while tensile strain measures elongation per unit length under tensile stress. Thus, the precision of volumetric strain in describing changes in volume per unit volume is what makes it the correct answer.