Dictionary
Contrast ratio
A measure of the difference between text and background luminance. WCAG AA requires at least 4.5:1 for normal body text.
In more detail
What It Means
Contrast ratio measures how different a text color is from its background, calculated from each color's relative luminance. WCAG AA requires at least 4.5:1 for normal body text and 3:1 for large text, so text stays readable for people with low vision or in bright light.
See it in context on the homepage: Accessibility: Design for Humans.