What is Impedance, admittance?

For a linear dipole in the sinusoidal steady state (receiver convention), the impedance is the ratio of complex amplitudes

whose modulus $Z = U/I$ relates the amplitudes and whose argument $\varphi = \varphi_u - \varphi_i$ is the phase lead of the voltage over the current. Its inverse $\underline Y = 1/\underline Z$ is the admittance (siemens). The real part of $\underline Z$ is its resistance, the imaginary part its reactance.