The input—output description of a system is applicable only when the system is initially relaxed. If a system is not initially relaxed, say at time to, then it depends also on the set of initial conditions at to, which we call the state. The state is the informati011 that, together with the input, determines uniquely the output. The state of a system at time to is the minimum amount of information needed about the system at time to, in addition to the system diagram (or system equations) and the system inputs between to and tl > to, so that all outputs at any time ti can be determined. The set of equations that describes the unique relations between the input, output, and state is called a dynamical equation.