Nyquist Frequency
Definition
The highest frequency you can faithfully record — exactly half your sampling rate. Sample slower and high notes fold into fakes.
Technical definition
f_N = f_s/2. The Nyquist–Shannon theorem states a signal must be sampled at at least twice its highest frequency to be reconstructed.
Example
Audio CDs sample at 44.1 kHz so they capture up to 22.05 kHz — just above the ~20 kHz limit of human hearing.
Common confusion
Sampling faster than Nyquist doesn't improve the captured band — it only extends the highest frequency you can capture. 96 kHz audio isn't 'more detailed' in the audible range.