What is audio aliasing?

Audio aliasing is a digital signal processing term that refers to how high frequencies (or harmonics) above the Nyquist limit are thrown back into your human hearing audible range (i.e., 20 to 20 kilohertz), particularly when using some of your favorite non-linear audio software plugins such as bus compressors, brickwall limiters, tape saturation, exciters, soft-clippers, virtual synthesizers, amp simulators, etc.

The maximum frequency content a DAW or an audio plug-in can handle is called its Nyquist limit. Therefore, the sample rate must be greater than or equal to two times the highest frequency content in the input signal.

When this rule ("Nyquist theorem") is not taken into consideration, it follows that undesirable signals will often be audible in the higher frequency range. And that phenomenon is called aliasing.

Furthermore, the human ear can only hear frequencies up to 20 kHz, and it's no coincidence a standard music CD has a sampling rate of 44.1 kHz, thus providing a bit-perfect representation of the frequencies humans can hear. This is done in an effort to reduce levels of aliasing.

Conclusion​

In other words, aliasing is often a result of loss of data translating to poor audio quality when a signal is sampled at a lower resolution than the original recording sampling rate. However, this can oftentimes be addressed through the technique of oversampling.