Definition
The easiest way to think of time invariance is to use the vertical and horizontal line test. x(t) is the horizontal axis and y(t) is the vertical axis.
x(t) --> system 1 --> y(t)
Time Invariant System
$ y=exp(x) $
This function passes both the horizontal and vertical line tests.
Non-Time Invariant System
$ y=x^2 $
This function fails the horizontal line test.
