Impedance spectroscopy (IS) is a general term that subsumes the small-signal measurement of the linear electrical response of a material of interest (including electrode effects) and the subsequent analysis of the response to yield useful information about the physicochemical properties of the system. Analysis is generally carried out in the frequency domain, although measurements are sometimes made in the time domain and then Fourier transformed to the frequency domain. IS is by no means limited to the measurement and analysis of data at the impedance level (e.g., impedance vs. frequency) but may involve any of the four basic immittance levels: thus, most generally, IS stands for immittance spectroscopy.