A component of video: the black and white or brightness element, of an image. It is often represented as Y, so the Y in Y,B-Y,R-Y, YUV, YIQ and Y,Cr,Cb is the luminance information of the signal.
In a color TV system the luminance signal is usually derived from the RGB signals originating from cameras, by a matrix or summation of approximately:
Y = 0.3R + 0.6G + 0.1B (based on ITU-R BT.601)
There are other similar equations from different TV standards. The precise values depend on the color primaries of the display standard used.