Convenient shorthand commonly, but incorrectly, used to describe the analog luminance and color difference signals in component video systems. Y is correct for luminance but U and V are, in fact, the two subcarrier modulation axes used in the PAL color coding system. Scaled and filtered versions of the B-Y and R-Y color difference signals are used to modulate the PAL subcarrier in the U and V axes respectively. The confusion arises because U and V are associated with the color difference signals but clearly they are not themselves color difference signals. Or could it just be because YUV trips off the tongue much more easily than Y, R-Y, B-Y?
See also: PAL