Refers to a ratio of sampling frequencies used to digitize the luminance (Y) and color difference components (R-Y, B-Y) of an image signal. The term 4:2:2 denotes that for every four samples of the Y luminance, there are two samples each of R-Y and B-Y color difference, allowing less chrominance (color) bandwidth in relation to luminance. This compares with 4:4:4 sampling where the same full bandwidth is given to all three channels, in this case usually sampled as RGB.
The term 4:2:2 originated from the ITU-R BT.601 digital video sampling where 4:2:2 sampling is the standard for digital studio equipment. The terms ‘4:2:2’ and ‘601’ are commonly (but technically incorrectly) used synonymously in TV. For SD the sampling frequency of Y is 13.5 MHz and that of R-Y and B-Y is each 6.75 MHz, providing a maximum color bandwidth of 3.37 MHz – enough for high quality chroma keying. For HD the sampling rates are 5.5 times greater, 74.25 MHz for Y, and 37.125 MHz for each of R-Y and B-Y.
The origin of the term is steeped in digital history and should strictly only be used to describe a specific format of standard definition digital television sampling. However, it is widely used to describe the sampling frequency ratios of image components (Y, B-Y, R-Y) of HD (1080-line HD is 5.5x SD, so 22:12:12 would be appropriate) and many other image formats including UHD.