How bright light from a surface or screen is, weighted by the eye's sensitivity to each wavelength.
Measured in candelas per square metre. WCAG's relative luminance is the Y value of a colour on a 0–1 scale; the eye is far more sensitive to green than to blue, so a pure blue has low luminance despite looking vivid.
Luminance is not the same as lightness. Lightness (L*) is a perceptual scale; equal steps in luminance do not look like equal steps.
Pure sRGB blue has a relative luminance of about 0.07, pure green about 0.72.
Contrast ratios, readable charts and greyscale printing all depend on luminance difference, not hue difference.