Time Invariant.
A system is time-invariant as long as the system shows certain fixed behaviors over time. For example, when x(t) shifts by a constant, y(t) should shift by the same constant.
$ y = x(t) $
$ x2 = x(t-t0) $
Then
$ y(t-t0) = x(t-t0) $
Also, the following should satisfy.
$ y = x(t) $
$ x2 = x(2t) $
Then
$ y(2t) = x(2t) $
