Nyquist frequency
Half the sample rate — the highest frequency a digital signal can carry before it turns into something else.
Harry Nyquist at Bell Labs showed in 1928 that two points per cycle are enough to describe an oscillation; Claude Shannon wrapped it up as the sampling theorem in 1949. At 44.1 kHz the limit sits at 22.05 kHz, just above what young ears can hear. That is not a coincidence — it is the whole reason for the number.
Content above the limit does not vanish. It folds back down and reappears as false tones lower in the spectrum, which is aliasing. Hence the low-pass filter in front of every ADC, and hence synths and distortion plugins oversample internally: a software distortion that generates harmonics above Nyquist otherwise sounds like a broken radio.