In physics, long-range order characterizes physical systems in which remote portions of the same sample exhibit correlated behavior.
Let us discuss this by a correlation function, namely the spin-spin correlation function:
This function is equal to unity when x = x' and decreases as the distance | x - x' | increases. Typically, it decays exponentially to zero at large distances, and the system is considered to be disordered. If, however, the correlation function decays to a constant value at large | x - x' | then the system is said to posses long-range order. If it decays to zero algebraically (i.e. as a polynomial) then we call it quasi-long-range order.
See also order-disorder
