Long-range order

Let us discuss this by correlation function,namely spin spin correlation function. G(x,x')= It decays with |x-x'|.If it decays to a constant value at large |x-x'| then it is said to posses the long range order.If it decays expontially to zero at large distance then we call it disordered . If it decays to zero algebrically (slowly) then we call it Quasi Long Range Order.