mean catalytic activity rate, ΔzΔt 

https://doi.org/10.1351/goldbook.M03775
The catalytic activity of a specified component in or moved from a system divided by the time during which the component was changed or moved. This kind of quantity should not be confused with rate of reaction.
Source:
PAC, 1979, 51, 2451. (Quantities and Units in Clinical Chemistry) on page 2475 [Terms] [Paper]