Porosity Φ, is a property that, in the field of geology and material sciences, is described as the ratio of void volume (Vv) to the total bulk volume of material (Vt). This void space consists of pores between grains or crystals, therefore, spaces within a rock body that are unoccupied by solid material .
Among the different terms and techniques involving the production of intentional voids in materials, emerges the concept of hierarchical design of materials. The term hierarchical design, as used in material science, is related to the control of material properties simultaneously at multiple scales. This hierarchical design produced by an intentional generation of voids in materials is often referred to as hierarchical porosity .
 Collins English Dictionary - Complete & Unabridged 10th Edition. Retrieved November 20, 2015.
 Gladysz, G. M., Chawla, K.K. “Voids in Materials”. 2015, Elsevier. P.65