The Free On-line Dictionary of Computing (30 December 2018):

Nyquist Theorem

    A theorem stating that when an analogue
   waveform is digitised, only the frequencies in the waveform
   below half the sampling frequency will be recorded.  In
   order to reconstruct (interpolate) a signal from a sequence of
   samples, sufficient samples must be recorded to capture the
   peaks and troughs of the original waveform.  If a waveform is
   sampled at less than twice its frequency the reconstructed
   waveform will effectively contribute only noise.  This
   phenomenon is called "aliasing" (the high frequencies are
   "under an alias").

   This is why the best digital audio is sampled at 44,000 Hz -
   twice the average upper limit of human hearing.

   The Nyquist Theorem is not specific to digitised signals
   (represented by discrete amplitude levels) but applies to any
   sampled signal (represented by discrete time values), not just
   sound.

   Nyquist
   (http://geocities.com/bioelectrochemistry/nyquist.htm)
   (the man, somewhat inaccurate).

   (2003-10-21)