A ratio of sampling frequency for a digitized video signal. The first number refers to the luminance part of the signal, the second two refer to the chroma (colour).
In this ratio, all parts of the signal are sampled equally, providing the best overall quality. All luminance and all chrominance information is included, as opposed to 4:2:2 and 4:1:1, in which the chrominance parts are reduced to save bandwidth.
Similar to 4:4:4, but with the addition of an extra channel for keying.