From AMS Glossary
(Also called Nyquist frequency.) The highest frequency that can be measured using discretely sampled data.
It is given by nf (rad s-1) = π/Δt, where nf is the Nyquist frequency and t is the time increment between observations. Stated another way, a minimum of two data points is needed to define a wave, hence the period of the smallest wave (i.e., the period corresponding to the Nyquist frequency) that can be measured is 2Δt. The word "folding" comes about because any frequencies that are higher than the Nyquist frequency in a continuous signal will be aliased or folded into lower frequencies when the signal is discretely sampled. To avoid this severe problem, the original signal must be filtered by analog or physical methods to remove all frequencies higher than the Nyquist frequency before the signal is sampled or digitized.