1. The ratio of the aggregate volume of interstices in a rock or soil to its total volume; generally stated as a percentage [10]. 2. The ratio, usually expressed as a percentage, of the total volume of voids of a given porous medium to the total volume of the porous medium [22]. 3. The volume percentage of the total bulk not occupied by solid particles [22]. See also porosity, effective; porosity, primary; porosity, secondary; porosity, tertiary.