Bond order is the number of bonds between a pair of atoms. For example in nitrogen N:::N the bond order is 3, in acetylene H:C:::C:H the bond order between the two carbon atoms is 3 and the C:H bond order is 1. Bond order gives an indication to the stability of a bond.

In a more advanced context bond order need not be an integer. A good example of this is bonds between carbon in the molecule benzene where the delocalized molecular orbitals contain 6 pi electrons over six carbons essentially yielding half a pi bond. Together with the sigma bond the bond order is 1.5. Furthermore bond orders of 1.1 for example may be spoken about in more complex terms and are essentially referring to bond strength relative to B.O.=1 bonds.

In Molecular Orbital Theory bond order is also defined as the difference between the number of bonding electrons and the number of antibonding electrons, divided by two. This often but not always yields the same result as more simple theories.

Bond order is an index of bond strength and is used extensively in valence bond theory.

In short: the bond order is the net amount of bonding between two atoms. BO = 1/2 ((no. of bonding electrons) - (no. of antibonding electrons))

The bond order concept is used e.g. in molecular dynamics bond order potentials.