The absolute volume of an object is the amount of space it takes up divided by the amount the object weighs. This tells how much space one unit of that object would fill. Oil, for example, is often measured in gallons per pound or in cubic meters per kilogram. The absolute volume of oil would show how many gallons of oil would fit in a single pound, or how many cubic meters would fit in a single kilogram.
Density is also related to absolute volume. Denser objects have more material packed tightly together inside them, and thus a highly dense object could take up a small amount of space, whereas an object taking up a lot of space might have a low density because the material it is composed of is not packed in tightly. An object might have a high density but low volume or a low density and a large volume.
Absolute volume is different from calculating regular volume in only one regard. Volume generally looks at how much space the object takes up in total, while absolute volume looks at how much space a single unit of that object takes up. Liquids, for example, expand out to fill their containers and thus have a lot of volume. In calculating the absolute volume of a liquid such as oil, it does not look at how much space all the oil takes up but instead looks out how much space one gallon or one cubic meter of the oil occupies. After finding the absolute volume of a material, it is then possible to calculate how much space different amounts would take up.
One way to test for the volume of a solid is to place it in a glass of water. The object displaces, or moves aside, the water where it is resting. Dropping a square plastic block, for example, into a glass of water causes the water to rise because the block is taking up some of the space and the water has to move out of its way. The amount of water the block displaces is the volume of the block, and this number divided by the block's own weight is its absolute volume.
Volume and absolute volume are important concepts in science and engineering. In engineering, scientists usually look at the material making up the object to calculate its volume but do not count the spaces between the particles. An object with a low density and a large volume may take up a lot of space because its particles are spread so far apart. A gas or liquid, for example, will continue to spread out and fill the containers they're placed in. Pouring a glass of water from a short, round jar into a tall, square glass will change its size and shape but it's still the same volume, or amount, of water.