These terms refer to the nyquist sampling theorem, but are used in different ways by different authors. They can be used to mean four different things depending on author:
- The highest frequency contained in a signal
- Twice the highest frequency contained in a signal
- the sampling rate
- One-half the sampling rate.
Nyquist sampling theorem
Often called just sampling theorem or sometimes shannon sampling theorem
A continuous signal (analog signal) can be properly sampled, only if it does not contain frequency components above one-half of the sampling rate. This means that with cd-audio's 44.1kHz sampling rate, the maximum frequency that can be represented properly is 22.05kHz.