Filtering
Linear time invariant systems have the property that if the input is described
as a sum of sinusoids, then the output is a sum of sinusoids of the same
frequency. Each sinusoidal component might be scaled differently, and each
may be subjected to a phase change, but the output will not contain any
sinusoidal components that are not also present in the input.
LTI systems are often used to remove sinusoidal components from a signal.
For this reason, an LTI system is often called a filter.