An abstraction for modelling circuits that assumes idealized resistors, capacitors, and inductors connected by ideal wires having no impedance. It is reasonably accurate when the wavelength of the signals of interest is much larger than the physical dimensions of the circuit. The resulting circuit theory can be developed using ordinary differential equations in which circuit parameters only vary over time. An example of a circuit element that cannot be treated this way is a transmission line, where the length is often larger than the signal wavelength.