Circuit theory is an approximation of electromagnetic theory, although circuit theory was developed independently based on experimental data. Compared with Maxwell's equations, which describes field distribution as a function of both time and space, circuit theory assumes no distance and time delay between any of the two nodes in the circuit. In other words, the wavelength of the signal is much longer than the dimension of the circuit (including each element); therefore, the spatial variation and time delay of the waveform could be ignored in the circuit analysis. To distinguish systems that are described by Maxwell's equations and circuit theory, those systems are defined as distributed and lumped circuits, respectively. As mentioned in earlier chapters, a circuit that has both types of systems is called a hybrid system.