Pre-exponential factor
In chemical kinetics, the pre-exponential factor or A factor is the pre-exponential constant in the Arrhenius equation, an empirical relationship between temperature and rate coefficient. It is usually designated by A when determined from experiment, while Z is usually left for collision frequency.
For a first-order reaction, it has units of s−1. For that reason, it is often called frequency factor.
The frequency factor, A, depends on how often molecules collide when all concentrations are 1 mol/L and on whether the molecules are properly oriented when they collide.
Values of A for some reactions can be found in here.
IUPAC Gold Book definition of pre-exponential factor
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Wikipedia