Water absorption is the mass of water a tile absorbs, expressed as a percentage of its dry mass. It is measured by boiling or by vacuum, as set out in ISO 10545-3, and it is the most direct indicator of how porous the body is.
The figure decides how a tile is classified and where it can be used. A low absorption goes with high strength, frost resistance and stain resistance; a high absorption goes with light, easily worked wall tiles.