Nuclear Energy Generation Rate (often ε) is the rate energy is released by Fusion per unit mass per unit time, e.g., Ergs per gram per second. Stellar Structure models use the concept, for sufficient simplicity, treating energy release of fusion as a function of composition, density, and Temperature. Such a function must deal with the different fusion reactions, e.g., the CNO Cycle, Proton-Proton Chains, and the Triple Alpha Process, which vary differently by temperature. The functions are often simplified (using series approximations) to incorporate temperature and density sensitivity as the product of a power of each.
The phrase Nuclear Reaction Rate is used for a factor in the analysis of nuclear energy generation, indicating the number of Nuclear Reactions (instances of fusion) per unit volume per second.