Luminance, better referred to as Luma when applied to video signals, represents the brightness or the "black and white" portion of the picture. Luma is typically paired with chroma in a composite video signal. The Luma portion of the signal represents the image without any color, while the chroma components represent the color hue and saturation information.

Mentioned in:
Video Detector, Detect Video Luminance Model 630 (Discontinued)
Saturation, defined in terms of video
IRE, defined in terms of video