In general, most studies refer to either preload or afterload. Afterload is usually referred to as a pressure load since that is the load against which the left ventricle must eject. Preload is often considered a volume load and is sometimes refers to the end diastolic volume. (Cf Circulation. 2010;122:993-1003.) So the physiology of unloading probably describes the reduction of aortic pressure or of the end diastolic volume.

But the details can be more complicated. Consider hypertension induced hypertrophy: The hypertrophy is developed because an increase in afterload, the pressure load. So here, unloading the ventricle may be simply an unloading in pressure. But volume unloading (diuretics) can be used to treat hypertension in patients.

So depending on the paper, you can consider unloading as either a reduction in pressure (afterload) or volume (preload), but there are caveats to such simple definitions.