What is meant by the term "absolute volume?" Volume should be volume, and I can't see how one can be more absolute than another.
This is a term that is applied to the ingredients of concrete or mortar. It specifically means the actual space occupied. With granular solids like cement and aggregate the apparent volume is larger than the absolute volume because the apparent volume includes spaces between particles. In designing mixes the absolute volumes of the amounts of all ingredients are calculated from the weight and specific gravities and then these are added to get the volume of the concrete made with those amounts of ingredients. As far as mixing water goes you are right—apparent volume and absolute volume are both the same as just plain volume.