Time Invariance
A time invariant system is one in which a time shifted input yields an output of the same time shift:
$ x(t-\delta) = y(t-\delta) $
Example of a time invariant system:
$ x(t) = \sin(t) $
Example of a time variant system:
$ x(t) = t\sin(t) $
In the variant example the amplitude of the signal depends on time so it is time variant
