Aliasing
Also known as "Under-Sampling". Aliasing is an undesired effect of under-sampling.
It occurs when a signal is not being sampled fast enough.
Mathematically, this occurs when:
$ w_s\leq 2w_n $
also as
$ \dfrac{2\pi}{T_s}\leq 2w_n $
for
$ w_s = \dfrac{2\pi}{T_s} $
This is closely tied to the Nyqust theorem, where if you are sampling beneath the Nyqust value, the signal will collide with itself, making it no longer perfectly recoverable.
