The Nyquist rate is the lowest rate at which a signal can be sampled without aliasing. It is equal to two times the highest frequency present in the signal.
An exception to this rule is bandpass sampling, in which aliasing is not necessarily a problem and sampling can successfully occur below the Nyquist rate. The term is associated with electronics engineer Harry Nyquist, though the theory is not entirely his creation.
In terms of audio and video, aliasing is an undesirable effect that can cause distortion or artifacts. A famous example in film is the wagon wheel effect, in which the spokes of a wheel appear to be moving too slowly or backwards.