The katal (symbol: kat) is that catalytic activity that will raise the rate of conversion by one mole per second in a specified assay system. It is a unit of the International System of Units (SI) used for quantifying the catalytic activity of enzymes (that is, measuring  the enzymatic activity level in enzyme catalysis) and other catalysts.
The unit katal is not attached to a specified measurement procedure or assay condition, but any given catalytic activity is: the value measured depends on experimental conditions that must be specified. Therefore, to define the quantity of a catalyst in katals, the catalysed rate of conversion (the rate of conversion in presence of the catalyst minus the rate of spontaneous conversion) of a defined chemical reaction is measured in moles per second. One katal of trypsin, for example, is that amount of trypsin which breaks one mole of peptide bonds in one second under the associated specified conditions.