Source : Free On-Line Dictionary of Computing

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.
     
        [Who was Nyquist?]
     
        (1998-12-30)