Porosity

Ratio of void volume and total volume of a porous material

Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure the "accessible void", the total amount of void space accessible from the surface.

This keyword could refer to multiple things. Here are some suggestions: