The minimum sampling frequency that will faithfully sample an analog signal so it can be accurately reconstructed from the digital result. This is always twice the maximum frequency of the signal being sampled.
In practice significantly higher sampling frequencies are used in order to stay well above the Nyquist frequency, where response drops to zero. This avoids the chance of producing aliens (unwanted artifacts) and the severe attenuation, according to a Sin x/x characteristic, that exists around the Nyquist point. For example in ITU-R BT.601 the maximum luminance frequency is 5.5 MHz and its sampling frequency is 13.5 MHz. Sin x/x shaped filters are used to counter the attenuation of the sampling system.