Before revision: Shift invariant system is the discrete equivalent of the time-invariant system ; defined such that if y(n) is the response of the system to x(n), then y( n-k ) is the response of the system to x( n-k ).Oppenheim, Schafer, 12
After revision: A shift invariant system is a discrete equivalent of the time-invariant system , defined such that if y(n) is the response of the system to x(n), then y( n–k ) is the response of the system to x( n–kThat is, in a shift-invariant system the contemporaneous response of the output variable to a given value of the input variable does not depend on when the input occurs; time shifts are irrelevant in this regard. ).Oppenheim, Schafer, 12