Aliasing is a (usually negative) side effect the way that computer audio is made up of discrete samples played back at a fixed sample rate. It can be caused by various DSP techniques and effects such as sample rate reduction, table lookups, or digital oscillators.
Aliasing can produce audible audio artifacts that are often undesirable. However, sometimes it can be desired, especially for harsh or extreme digital sounds.
The highest frequency that can be represented via samples for a given SR samples/second is the [nyquist-frequency]: nyquist (hz) = SR / 2
There are two main ways of counteracting aliasing: