 Conversely, in such cases the conductivity must be expressed as a complex number (or even as a matrix of complex numbers, in the case of anisotropic materials) called the "admittivity".Admittivity is the sum of a real component called the conductivity and an imaginary component called the susceptivity.An alternative description of the response to alternating currents uses a real (but frequency-dependent) conductivity, along with a real permittivity.