Aliasing
Aliasing is the effect that makes the wheel of a rolling wagon appear to be going backwards.
The backward rolling effect is introduced because the movie's frame rate isn't adequate to describe
the rotational frequency of the wheel, and our eyes are deceived by the misinformation.
An example from the audio world:
Consider a 48kHz digital audio system recording a steadily rising sine wave tone.
At lower frequency, the tone is sampled with many points per cycle.
As the tone rises in frequency, the cycles get shorter and fewer and fewer points are available to describe it.
At a frequency of 24 KHz, only two sample points are available per cycle, and we are at the limit of what
Nyquist says we can do. Still, those two points are adequate, in a theoretical world, to recreate the tone
after conversion back to analog and low-pass filtering.
However, if the frequency increases, the number of samples per cycle is not adequate
to describe the waveform, and the inadequate description is equivalent to one describing a lower
frequency tone, whereby aliasing is introduced.