is a rate constant,
the frequency factor
and
the activation energy,
and 
sometimes exhibit compensation, i.e. they change in the same direction with change
in catalyst for a given reaction or with change in reaction for a given catalyst.
Source:
PAC, 1976, 46, 71
(Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix
II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II:
Heterogeneous Catalysis)
on page 82