The energy required to break a given type of bond between atoms in certain valence states. An averaged bond energy is commonly derived by dissecting the heat of atomization of a molecule into contributions of individual bonds. For molecules with localized bonds, the heats of atomization (formation) are usually well approximated by the sum of pertinent averaged bond energies.
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1927 (https://doi.org/10.1351/pac199971101919)