Lumped element circuits are electronic circuits where the electrical signals are treated as being confined to discrete points, and the circuit components are physically small compared to the length of the signals being transmitted. In other words, the electrical behavior of the circuit is assumed to be localized in small components such as resistors, capacitors, and inductors, rather than being distributed over a large area. the behavior of lumped element circuits can be fully analyzed using circuit theory, which is a simplified approximation of electromagnetic field theory.

On the other hand, distributed element circuits are electronic circuits where the electrical signals are treated as being spread out over a larger area, and the circuit components have physical dimensions that are comparable to the length of the signals being transmitted. In this type of circuit, the behavior of the circuit is not confined to discrete points or components, but is distributed over a larger area. Some examples of device that needs distributed circuit consideration are transmission line, waveguide, and antenna. In distributed element circuits, the behavior of the circuit cannot be fully analyzed using circuit theory, and instead requires more fundamental and thorough consideration of electromagnetic wave/field theory.

Fortunately, we can model distributed element circuit with lumped circuit consideration to a certain extent. This modeling process is done by defining a set of lumped circuit components for a unit length of a given distributed circuit as shown in the figure above. we will explore more detailed derivation and examples of such distributed circuit in the next chapters.