The distance between atomic centers involved in a chemical bond. The notion of bond length is defined differently in various experimental methods of determination of molecular geometry; this leads to small (usually 0.01-0.02 Å)
differences in bond lengths obtained by different techniques. For example, in gas-phase electron-diffraction experiments, the bond length is the interatomic distance averaged over all occupied vibrational states at a given temperature. In an X-ray crystal structural method, the bond length is associated with the distance between the centroids of electron densities around the nuclei. In gas-phase microwave spectroscopy, the bond length is an effective interatomic distance derived from measurements on a number of isotopic molecules, etc. A number of empirical relationships between bond lengths and bond orders in polyatomic molecules were suggested, see, for example, fractional bond number (the Pauling's bond order).
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1927 (https://doi.org/10.1351/pac199971101919)