The katal is not used to express the rate of a reaction; that is expressed in units of concentration per second (or moles per liter per second). Rather, it is used to express catalytic activity which is a property of the catalyst. The katal is invariant of the measurement procedure, but the numerical quantity value is not and depends on the experimental conditions. Therefore, in order to define the quantity of a catalyst, the rate of conversion of a defined chemical reaction is specified as mols reacted per second. One katal of trypsin, for example, is that amount of trypsin which breaks a mole of peptide bonds per second under specified conditions.
The name "katal" has been used for decades, and the unit became an official SI unit in 1999. The name comes from the Ancient Greek κατάλυσις (katalysis or katalusis), meaning "dissolution", which is the same origin as the word "catalysis" itself comes from.