What is Bond order?

Example 14.15 (Nitrogen and oxygen ions)

$\ce{N2}$ ($I = 15.58\,\mathrm{eV}$) loses a bonding electron: $D_0(\ce{N2+}) = 9.76 + 14.53 - 15.58 = 8.71\,\mathrm{eV}$, weaker than $\ce{N2}$. $\ce{O2}$ ($I = 12.07\,\mathrm{eV}$) loses a $\pi^*$ electron: $D_0(\ce{O2+}) = 5.12 + 13.62 - 12.07 = 6.66\,\mathrm{eV}$, stronger than $\ce{O2}$, in line with bond orders 2.5 and 2.5 against 3 and 2.