The average value of the gas-phase bond dissociation energies (usually at a temperature of 298 K) for all bonds of the same type within the same chemical species. The mean bond energy for methane, for example, is one-fourth the enthalpy of reaction for:
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Tabulated bond energies are generally values of bond energies averaged over a number of selected typical chemical species containing that type of bond.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1090 (https://doi.org/10.1351/pac199466051077)