Aliasing

Aliasing is when frequencies above the Nyquist frequency ($f_s / 2$) get sampled and "fold back" as false, lower frequencies that were never in the original sound.

For example, at 44.1 kHz a 25 kHz tone is heard as 19.1 kHz ($44.1 - 25$).

Aliases usually aren't harmonically related to the source, so they sound harsh. Once sampled, an alias can't be told apart from a real frequency, so it has to be prevented before sampling with an anti-aliasing filter.

Aliasing can also happen inside the computer, whenever a process creates new frequencies above Nyquist: distortion, clipping, or downsampling without a filter. Plugins prevent this with oversampling.