A system is **time-invariant** if a time shift in the input signal results in an equal time shift in the output signal. $S(x(n)) = y(n) \xrightarrow[\text{time-invariant}]{} S(x(n-n_0)) = y(n-n_0)$ Examples of time-invariant systems: 1. $y(t) = x(t-2)$ 2. $y(n) = sin(x(n))$ Examples of time-varying systems: 1. $y(t) = x(2t)$ 2. $y(n) = n x(n)$