What is the definition of porous materials? Is there a consensus on scientific definition of porous materials? There are many definitions out there which do not seem accurate enough:

from wikipedia (accessed July 2017):
A porous medium or a porous material is a material containing pores (voids).

-problem : Then almost all materials are porous, since there is some sort of void in everything.

from Merriam Webster (website accessed July 2017):
Containing pores, permeable to fluids, capable of being penetrated.

-problem : A piece of tubing would be porous since fluid passes through it.

At what scale do we consider the pores to qualify a material as porous? How much does the permeability matter in a formal definition? Is there a convention on defining pores and porous materials that is agreed upon by scientific organizations like IUPAC or ASTM etc?
I have used the following definition in my presentations:

"Porosity is where a material has accessible voids which are permeable to liquids or gases".

Another definition:

"a  solid with pores, i.e. cavities, channels or interstices, which are deeper than they are wide."

IUPAC also defines different types of porosity based on the diameters of the pores:


Micropores: d &lt; 2 nm
Mesopores: 2 &lt; d &lt; 50 nm
Macropores: d > 50 nm 


M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K. S. W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 2015, 87, 1051.