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A porous medium or a porous material is a material containing pores. The skeletal portion of the material is often called the "matrix" or "frame". The pores are typically filled with a fluid. The skeletal material is usually a solid, but structures like foams are often also usefully analyzed using concept of porous media.
A porous medium is most often characterised by its porosity. Other properties of the medium can sometimes be derived from the respective properties of its constituents and the media porosity and pores structure, but such a derivation is usually complex. Even the concept of porosity is only straightforward for a poroelastic medium.
Often both the solid matrix and the pore network are continuous, so as to form two interpenetrating continua such as in a sponge. However, there is also a concept of closed porosity and effective porosity, i.e. the pore space accessible to flow.
Many natural substances such as rocks and soil , zeolites, biological tissues , and man made materials such as cements and ceramics can be considered as porous media. Many of their important properties can only be rationalized by considering them to be porous media.