The conversion from a continuously changing measure to a series of measured values at discrete time instances is called sampling. The rate (or number of measurements per second) of which the sampling is done, is along with the quantization depth the most important quality factor of digital recording equipment. If it is set too low, disturbing artifacts occur. A CD quality recording is recorded with a sampling rate of 44 100 samples per second. We say that the sampling frequency is 44 100 Hertz (or short Hz).
In fact, all frequencies above half the sampling frequency, which is known as the Nyquist frequency, are substituted by frequencies below the Nyquist frequency. This effect is called aliasing. To avoid aliasing a sampling system contains of a low pass filter which ideally filters out all frequencies above the Nyquist frequency and leaves all frequencies below unaffected. In the case of the audio CD, the highest frequency that can theoretically be recorded is 22 050 Hz.