The conversion in a catalytic reaction performed under constant conditions of reaction often decreases with time of run or time on stream. This phenomenon is called catalyst deactivation or
catalyst decay.
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 83