What do you mean by the enthalpy of dissociation?

Ans.  The bond dissociation enthalpy is defined as the energy needed to dissociate or break one mole of the bond to give separated atoms, and one must note that everything must be in the gas state. 

For example- breaking up 1 mole of gaseous hydrogen chloride molecules into separate gaseous hydrogen and chlorine atoms takes 432 kilojoules.