Nyquist frequency

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.