A quantity which approximates to the @C00977@ associated with certain @N04262@ profiles of diffuse layers. \(\xi \) is defined as the separation between two interacting entities (molecules, @M03667@ segments, ions, spins), either in bulk or interfacial regions, beyond which an appropriate function, referred to as pair correlation function, either decays (e.g. to \(\frac{1}{e}\) of an 'initial' value) or vanishes. A characteristic length must be confused neither with the effective thickness of the @I03085@, nor with the range (effective distance) of the @I03098@ (pair) potential. This range is defined by the distance over which this potential and the corresponding correlation function both vanish.