Porosity

That part of a rock that contains pore spaces without regard to size, shape, interconnection, or arrangement of openings. It is expressed as percentage of total volume occupied by spaces.

The ratio of pore space to total volume. That portion of a cubic foot of soil that is air space and could therefore contain moisture.


Source: Handbook of water and wastewater treatment plant operations

Wikipedia:  https://en.wikipedia.org/wiki/Porosity