Contrast Ratio
Contrast ratio describes the calculated difference in luminance between two colours. At level AA, WCAG requires at least 4.5:1 for text and 3:1 for large text (success criterion 1.4.3), and likewise 3:1 for meaningful graphics and user interface components (success criterion 1.4.11).
In practice
Large text means at least 18 point, or 14 point bold (roughly 24 px, or 18.66 px bold). Level AAA requires 7:1 for text (success criterion 1.4.6). Free contrast checkers show quickly whether a colour combination meets the requirement. Sufficient contrast helps not only people with a visual impairment but everyone reading in poor light or on a reflective screen.