**To be very abstract, a system is something that gets an input and has an output.** The numbers of inputs and outputs can vary so we will differentiate between a Single Input Single Output System (SISO-System) and a Multiple-Inputs Multiple-Output Systems (MIMO-System).

In systems engineering, everything can be broken down into 3 basic elements or any combination of them. These elements are the P-, the I- and D-element. The indices P, I and D correspond to the proportionality of the system's output to input. The output y of a system is proportional

- directly to the input u (P).
- to the integral of the input u (I).
- to the derivative of the input u (D).