Describing a linear system with the additional property that the only effect delaying the time at which an input signal is applied is to have the same output signal with the same delay. If y(t) is the time-varying output of the system as a function of time-varying input x(t), then delaying the input by d, (x(t+d)) simply results in a similar delay to the output (y(t+d)). LTI systems can be completely modelled by determining their impulse response.