In signal processing, the **Nyquist frequency** (or **folding frequency**), named after Harry Nyquist, is a characteristic of a sampler, which converts a continuous function or signal into a discrete sequence. In units of cycles per second (Hz), its value is one-half of the sampling rate (samples per second).^{[1]}^{[2]}^{[A]} When the highest frequency (bandwidth) of a signal is less than the Nyquist frequency of the sampler, the resulting discrete-time sequence is said to be free of the distortion known as aliasing, and the corresponding sample-rate is said to be above the Nyquist rate for that particular signal.^{[3]}^{[4]}

In a typical application of sampling, one first chooses the highest frequency to be preserved and recreated, based on the expected content (voice, music, etc.) and desired fidelity. Then one inserts an anti-aliasing filter ahead of the sampler. Its job is to attenuate the frequencies above that limit. Finally, based on the characteristics of the filter, one chooses a sample-rate (and corresponding Nyquist frequency) that will provide an acceptably small amount of aliasing.

In applications where the sample-rate is pre-determined, the filter is chosen based on the Nyquist frequency, rather than vice versa. For example, audio CDs have a sampling rate of 44100 samples/sec. The Nyquist frequency is therefore 22050 Hz. The anti-aliasing filter must adequately suppress any higher frequencies but negligibly affect the frequencies within the human hearing range; a filter that preserves 0–20 kHz is more than adequate for this.

Main article: Aliasing |

In this example, *f*_{s} is the sampling rate, and 0.5 *f*_{s} is the corresponding Nyquist frequency. The black dot plotted at 0.6 *f*_{s} represents the amplitude and frequency of a sinusoidal function whose frequency is 60% of the sample-rate. The other three dots indicate the frequencies and amplitudes of three other sinusoids that would produce the same set of samples as the actual sinusoid that was sampled. Undersampling of the sinusoid at 0.6 *f*_{s} is what allows there to be a lower-frequency alias. If the true frequency were 0.4 *f*_{s}, there would still be aliases at 0.6, 1.4, 1.6, etc.

The red lines depict the paths (loci) of the 4 dots if we were to adjust the frequency and amplitude of the sinusoid along the solid red segment (between *f*_{s}/2 and *f*_{s}). No matter what function we choose to change the amplitude vs frequency, the graph will exhibit symmetry between 0 and *f*_{s}. This symmetry is commonly referred to as **folding**, and another name for *f*_{s}/2 (the Nyquist frequency) is **folding frequency**.^{[5]}

Early uses of the term *Nyquist frequency*, such as those cited above, are all consistent with the definition presented in this article. Some later publications, including some respectable textbooks, call twice the signal bandwidth the Nyquist frequency;^{[6]}^{[7]} this is a distinctly minority usage, and the frequency at twice the signal bandwidth is otherwise commonly referred to as the Nyquist rate.