Given a system with a time-dependent output function and an input function , the system is system is time invariant if a time-delay on the input directly equates to a time-delay of the output function.

Time_invariance_block_diagram_for_a_SISO_system.png

We find that applying delay to the input or output gives us the same output, so the system is time invariant.

If a time-invariant system is also linear, it is called a linear time-invariant system.