Circuit elements are the basic building blocks of an electric circuit. Essentially, an electric circuit is the interconnection of these elements. Within electric circuits, one can find two types of elements: passive and active. Active elements have the ability to generate energy, whereas passive elements do not. Passive elements include components like resistors, capacitors, and inductors, while active elements typically encompass generators, batteries, and operational amplifiers.

The most crucial active elements are voltage or current sources, which generally provide power to the connected circuit. There are numerous ways to categorize circuit elements. For instance, distinguishing linear models from nonlinear models is crucial because circuits composed entirely of linear circuit elements are simpler to analyze than those with some nonlinear elements.

An element or circuit is considered linear if the element's excitation and response meet certain conditions. Both superposition and homogeneity properties are satisfied by a linear element. Consider a scenario where the excitation is the current (i) and the response is the voltage (v). If the element is subjected to a current (i₁), it delivers a response (v₁). Similarly, when the element is exposed to another current (i₂), it elicits a response (v₂).

Any circuit element that fails to meet either the superposition or the homogeneity principle is classified as nonlinear. Thermistors are examples of such nonlinear elements.