It is the rate of water absorption by the porcelain material. Well, as you search for the best tiles that suit your needs, it is best to ask about the porosity rate, which could fit in any of the following categories: Impervious (a poor absorber of water) Vitreous Semi-vitreous Non-vitreous (absorbs the most) For the outdoors, like the swimming pool area, ensure the tiles are non-porous.