奈奎斯特频率（Nyquist频率）是离散信号系统采样频率的一半，因哈里·奈奎斯特（Harry Nyquist）或奈奎斯特－香农采样定理得名。采样定理指出，只要离散系统的奈奎斯特频率高于被采样信号的最高频率或带宽，就可以避免混叠现象。

从理论上说，即使奈奎斯特频率恰好大于信号带宽（但不可相等），也足以通过信号的采样重建原信号。但是，重建信号的过程需要以一个低通滤波器或者带通滤波器将在奈奎斯特频率之上的高频分量全部滤除，同时还要保证原信号中频率在奈奎斯特频率以下的分量不发生畸变，而这是不可能实现的。在实际应用中，为了保证抗混叠滤波器的性能，接近奈奎斯特频率的分量在采样和信号重建的过程中可能会发生畸变。因此实际应用中信号带宽并不能无限接近奈奎斯特频率，具体的情况要看所使用的滤波器的性能。

The Nyquist frequency, named after electronic engineer Harry Nyquist, is half of the sampling rate of a discrete signal processing system. It is sometimes known as the folding frequency of a sampling system.  An example of folding is depicted in Figure 1, where 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 (f_s). 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. The symmetry about 0.5 f_s is referred to as folding.