A number from 1:1 to 21:1 describing how different two colours are in lightness, used to judge whether text and interface elements can be read.
WCAG defines it as (L1 + 0.05) / (L2 + 0.05), where L1 and L2 are the relative luminances of the lighter and darker colour. Normal text needs at least 4.5:1 and large text 3:1 for level AA; graphical objects and interface components need 3:1.
It measures lightness difference only. Two colours of very different hue can have a contrast ratio near 1:1 and be hard to tell apart for many readers.
Black on white is 21:1; mid grey #777777 on white is about 4.48:1, just short of AA for body text.
It is the testable core of accessible colour in digital products, and the reason hue alone never carries meaning safely.