Porcelain tile is a ceramic tile whose body has been fired until it is almost entirely vitrified. Its water absorption is 0.5% or less, which places it in group BIa of EN 14411 and ISO 13006, the lowest absorption group for dry-pressed tiles.
The body is made from kaolin, ball clay, feldspar and quartz, pressed at high pressure and fired at around 1,200 °C. The result is hard, strong and stain resistant, and its low absorption is why porcelain is used on floors, facades, terraces and in wet rooms as well as on walls. Frost resistance is a tested property rather than a given: for a surface that will freeze, check that the manufacturer declares it for the tile in hand.