键能通常指在标准状态下气态分子拆开成气态原子时，每种化学键所需能量的平均值。

In chemistry, bond energy (E) or bond enthalpy (H) is the measure of bond strength in a chemical bond. IUPAC defines bond energy as 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. For example, the carbon-hydrogen bond energy in methane H(C–H) is the enthalpy change involved with breaking up one molecule of methane into a carbon atom and 4 hydrogen radicals, divided by 4. Tabulated bond energies are generally values of bond energies averaged over a number of selected typical chemical species containing that type of bond.
Bond energy (E) or bond enthalpy (H) should not be confused with bond-dissociation energy. Bond energy is the average of all the bond-dissociation energies in a molecule, and will show a different value for a given bond than the bond-dissociation energy would. This is because the energy required to break a single bond in a specific molecule differs for each bond in that molecule. For example, methane has 4 C-H bonds and the bond-dissociating energies are 435 kJ/mole for D(CH3-H), 444 kJ/mole for D(CH2-H), 444 kJ/mole for D(CH-H) and 339 kJ/mole for D(C-H). Their average, and hence the bond energy is 414 kJ/mole, even though not a single bond required specifically 414kJ/mole to be broken.