Katal is the SI derived unit for catalytic activity. It is defined as moles per second. The name katal had been used for this unit for decades, but did not become an official SI derived unit until 1999 with Resolution 12 of the 21st CGPM, on the recommendation of the International Federation of Clinical Chemistry and Laboratory Medicine . The formal adoption of the katal is hoped to discourage the use of a non-SI unit called "unit", symbol "U", defined as micromoles per minute. "Units" are more commonly used than the katal in practice at present, but their definition lacks coherence with the SI system.
The katal is not used to express the rate of a reaction; that is expressed in moles per second. Rather, it is used to measure the quantity of a catalyst. One katal of trypsin, for example, is that amount of trypsin which breaks a mole of peptide bonds per second.