Absorption rate is a measurement of how much water a fired, unglazed clay body test bar takes on by weight, expressed as a percentage of the bar's dry weight. It is the standard practical way potters and manufacturers describe how vitrified a clay body has actually become at a given firing temperature, since vitrification itself is a matter of degree rather than a single sharp threshold.
The test is straightforward in principle: a fired, unglazed test bar is weighed dry, submerged in water (usually by boiling to help water penetrate the body's pore structure) for a set period, then weighed again wet. The percentage weight gained is the absorption rate. A low absorption rate indicates a dense, well-vitrified body with little open pore space remaining; a higher rate indicates a more porous body.
Earthenware bodies typically show a much higher absorption rate than stoneware or porcelain fired to their own maturity points, which is the practical reason unglazed earthenware is not considered watertight while a well-vitrified stoneware body can be. Absorption rate is one of the standard figures manufacturers publish alongside a clay body's recommended firing range; see cone charts for how that range is specified.
As a general sense of scale rather than a precise rule: earthenware often absorbs somewhere in the range of 5 to 15% of its dry weight in water, a well-vitrified stoneware body is usually down in the low single digits, and a fully vitrified porcelain body can drop below 1%. Exact figures vary by specific clay and firing temperature, which is why manufacturers test and publish their own numbers rather than potters relying on rules of thumb for anything that actually needs to hold liquid reliably.
The boil-and-weigh procedure described above is standardized: laboratories and manufacturers run it as ASTM C373, the test method that specifies water absorption alongside two related figures calculated from the same test bar, bulk density and apparent specific gravity, rather than absorption in isolation. Following a single standardized procedure is what makes an absorption figure from one manufacturer meaningfully comparable to another's, rather than each lab's own informal variant of the boil test.