A linear circuit is an electric circuit in which, for a sinusoidal input voltage of frequency f, any output of the circuit (current through any component, voltage across any component, etc.) is also sinusoidal with frequency f. Note that the output need not be in phase with the input.

Another (equivalent) way of defining a linear circuit is any electronic circuit whose output is a linear transform of its input, where linear means that f(ax_1+bx_2) = af(x_1)+bf(x_2). Some examples are amplifiers, differentiators, and integrators, or any circuit composed exclusively of ideal resistors, capacitors, inductors, op-amps (in the "non-saturated" regime), and other "linear" circuit elements.

Linear circuits can be analyzed using the superposition principle, which, for example, allows Fourier analysis to be used.