1. A system whose output is linearly related to its input. If a linear system is excited by a sine wave of frequency f1, the output will contain only the frequency f1; the amplitude and phase may be changed, however. The rule of scaling (if A results in B, then kA results in kB, k being any constant) and the rule of superposition (if A results in B and C results in D, then A+C results in B+D) apply. 2. An electrical circuit whose impedance is independent of applied voltage (or current).