 The Nyquist frequency, or Nyquist rate, is the minimum sampling rate required to accurately reconstruct a continuous signal from its sample representation. A consequence of sampling a signal is that you are throwing away some of the information about it. This causes the frequency response of the signal to reflect around half of the sampling rate. If the signal contains a frequency that is greater than this Nyquist frequency, a phenomenon known as aliasing occurs. This is what it sounds like. Aliasing occurs because there are enough samples in the signal to represent the higher frequency. Once sampled, both the higher frequency and its companion frequency below the Nyquist rate look exactly the same. Therefore, the higher frequency gets aliased given the value of the lower frequency of the pair. To avoid this problem, care must be taken to set the sampling rate to greater than at least twice the highest frequency of interest in the signal and filter out anything higher before the signal is sampled.