Aliasing

Aliasing is inharmonic distortion that appears when a digital signal contains frequencies above half the sample rate (Nyquist), which reflect back down into the audible range.

In oscillators it shows up as metallic, out-of-tune artifacts that worsen as pitch rises, because higher notes push a waveform's partials past Nyquist. Wavetable engines control it by band-limiting playback per pitch, and nonlinear processing such as wavefolding re-introduces it unless it is oversampled.