In chemical kinetics, the preexponential 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. Values of A for some reactions can be found in here.
References
IUPAC Goldbook definition of preexponential factor IUPAC Gold Book definition of pre-exponential factor
