The Nyquist frequency is a term used in signal theory. It is also known as the Nyquist limit of half the sampling frequency and is defined as:
A similar concept is the Nyquist rate, that is the Nyquist frequency corresponding to the sampling rate. The term was coined by Claude Elwood Shannon and named after Harry Nyquist.
After the underlying Nyquist - Shannon sampling all fractions within a signal have frequencies smaller than the Nyquist frequency, so that the sampled signal can be reconstructed with arbitrary precision:
The sampling theorem states that the clock frequency of the point-wise sampling of the original signal must be more than twice as high as the highest frequency contained in the original signal:
If this criterion is not observed, arising non-linear distortions that are also referred to as aliasing. These can not filter out again.