What do you mean by: (i) Bond length  (ii) Bond enthalpy   (iii) Bond order?

(i) Bond length:  Bond length is defined as the equilibrium internuclear separation distance of the bonded atoms in a molecule. Each atom of the bonded pair contributes to the bond length. 

In the case of a covalent bond, the contribution from each atom is called covalent radius of that atom.
The bond length in a covalent molecule AB                    
  R = r_A + r_B
where R is the bond length; r_Aand r_B are the covalent radii of atoms A and B respectively.
(ii) Bond enthalpy: The strength of a chemical bond is measured as the bond dissociation enthalpy. It is the enthalpy required to break a particular bond in one mole of a gaseous molecule. For example for a homonuclear diatomic molecule H₂, we have,

Similarly for a heteronuclear diatomic molecule HCl, we have,

For molecules like O₂ and N₂ containing double and triple bonds:


(iii) Bond Order: According to Lewis concept of a covalent bond, the bond order is given by the number of bonds between the two atoms in a molecule. For example,
Bond order in H₂ (H – H) = 1
Bond order in O₂ (O = O) = 2)
Bond order in N₂ (N ≡ N) =3
Similarly, the bond order in CO (three shared electron pairs between C and O) is 3.
Isoelectronic molecules and ions have identical bond order; For example, F₂ and O₂^2–have bond order 1. N₂, CO and NO⁺ have bond order 3.
It should be noted that with the increase in bond order, bond enthalpy increases and bond length decreases.