The Hildebrand solubility parameter is the square root of the cohesive energy density:
The cohesive energy density is the amount of energy needed to completely remove unit volume of molecules from their neighbours to infinite separation (an ideal gas). This is equal to the heat of vaporization of the compound divided by its molar volume in the condensed phase.  In order for a material to dissolve, these same interactions need to be overcome, as the molecules are separated from each other and surrounded by the solvent.  In 1936 Joel Henry Hildebrand suggested the square root of the cohesive energy density as a numerical value indicating solvency behavior.  This later became known as the "Hildebrand solubility parameter".  Materials with similar solubility parameters will be able to interact with each other, resulting in solvation, miscibility or swelling.