Nyquist frequency

[3][4] In a typical application of sampling, one first chooses the highest frequency to be preserved and recreated, based on the expected content (voice, music, etc.)

Finally, based on the characteristics of the filter, one chooses a sample rate (and corresponding Nyquist frequency) that will provide an acceptably small amount of aliasing.

No matter what function we choose to change the amplitude vs frequency, the graph will exhibit symmetry between 0 and  fs.

[5] Early uses of the term Nyquist frequency, such as those cited above, are all consistent with the definition presented in this article.

Some later publications, including some respectable textbooks, call twice the signal bandwidth the Nyquist frequency;[6][7] this is a distinctly minority usage, and the frequency at twice the signal bandwidth is otherwise commonly referred to as the Nyquist rate.

Typical example of Nyquist frequency and rate. To avoid aliasing, the sampling rate must be no less than the Nyquist rate of the signal; that is, the Nyquist rate of the signal must be under double the Nyquist frequency of the sampling.
The black dots are aliases of each other. The solid red line is an example of amplitude varying with frequency. The dashed red lines are the corresponding paths of the aliases.