Time Invariant Filter
A filter which [see page 3, produces] the same output for the same input, regardless of delays.
That is to say if an input signal x[0] produces y[0], then if x[0] is delayed by 2 seconds than y[0] is also delayed by 2 seconds. That is to say, the filter doesn't depend on the timing of the input to determine its output.
x[n] => y[n]
x[n-n_0] => y[n-n_0]