If you pretend that the signals in electric circuits are of the form A*exp(s*t), where A and s are constants, and t is time, then derivatives of the signal and integrations of

… the signal are always just a scaled version of the signal. So, the ratio of voltage to current, when the signals are of this form, is just a (possibly complex) constant. This number is called impedance, and it depends on the value of the constant "s" in the signal. Impedance is a term used to signify a device that react to the flow of electrons in a negative way. It can be of complex passive devices or active devices. Answer Impedance (symbol: Z ), expressed in ohms , is the opposition to the flow of alternating current by a resistive-reactive load. It is the vector sum of resistance and reactance. (MORE)