The Nyquist frequency, named after the Swedish-American engineer Harry Nyquist, is half the sampling frequency of a discrete signal processing system. It is sometimes known as the folding frequency of a sampling system.
When the continuous function being sampled contains no frequencies equal or higher than the Nyquist frequency, all the aliases caused by sampling occur above the Nyquist frequency. The term aliasing usually refers to the case where some original frequency components have aliases below Nyquist. That often causes distortion when a continuous function is subsequently reconstructed from samples.
The Nyquist frequency should not be confused with the Nyquist rate, which is the lower bound of the sampling frequency that satisfies the Nyquist sampling criterion for a given signal or family of signals. This lower bound is twice the bandwidth or maximum component frequency of the signal. Nyquist rate, as commonly used with respect to sampling, is a property of a continuous-time signal, not of a system, whereas Nyquist frequency is a property of a discrete-time system, not of a signal. While the domain of the signals is commonly time, leading to a Nyquist frequency in Hertz, this does not have to be the case; for example, an image sampling system has a Nyquist frequency expressed in units of reciprocal length, such as cycles per meter.
Read more about Nyquist Frequency: The Aliasing Problem, Other Meanings
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