Define the bond length.

(N/A) Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule.
Bond lengths are expressed in terms of $\mathring{A}$ $(10^{-10} \ m)$ or picometer $(10^{-12} \ m)$ and are measured by spectroscopic $X$-ray diffraction and electron-diffraction techniques.
In an ionic compound,the bond length is the sum of the ionic radii of the constituting atoms $(d = r_+ + r_-)$. In a covalent compound,it is the sum of their covalent radii $(d = r_A + r_B)$.