A thermodynamic theory of polymer solutions, first formulated independently by Flory and by Huggins, in which the thermodynamic quantities of the solution are derived from a simple concept of combinatorial entropy of mixing and a reduced Gibbs energy parameter, the 'χ parameter'.
Source:
Purple Book, 1st ed., p. 59 (http://old.iupac.org/publications/books/author/metanomski.html)