Ordinarily, a reaction in which $\ce{A + H2}$ gives $\ce{B}$.
For example,
The term may also be used for cleavage of bonds other than $\ce{A}$, e.g.
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 86 [Terms] [Paper]
For example, 
The term may also be used for cleavage of bonds other than $\ce{A}$, e.g. 
