For most systems of interest, the output at time to depends not only on the input applied at to, but also on the input applied before and after to. If an input x(t), t > to, is applied to a system, unless we know the input applied before to, the output y(t), t > to, is generally not uniquely determinable. For different inputs applied before to, we will obtain different output y(t), t to, although the same input x(t), t > to, is applieŒ Hence, in developing the input—output description, before an input is applied, the system must be assumed to be relaxed or at rest, and that the output is excited solely and uniquely by the input applied. This definition is general and it is valid for an arbitrary system, continuous-time, discrete-time, and so on. If the concept of energy is applicable to a system, the system is said to be relaxed at time to if no energy is stored in the system at that instant. A system is said to be relaxed at time to if the output y(t), t > to, is solely and uniquely excited by the input x(t) defined for t > to.