Define Time and Shift Invariant?
Time and shift invariant means that the system characteristics and shift do not change with time, i.e. the system output is independent of the time the input is applied. For example, if the input x[n] gives the output y[n], then the input x[n - k] will give the output y[n - k]:
x[n] → y[n]
x[n - k] → y[n - k]
that is a delay in the input causes a delay by the same amount in the output signal.