Source : WordNet®

Nyquist frequency
     n : (telecommunication) twice the maximum frequency occurring in
         the transmitted signal

Source : Free On-Line Dictionary of Computing

Nyquist frequency
     
         The highest frequency that can be represented in a
        digital signal of a specified {sampling frequency}.  It is
        equal to one-half of the sampling rate.
     
        See {Nyquist Theorem}.
     
        (2001-06-06)