fonts/cc-by-sa/knowledge/glossary/terms/contrast/content.md

Contrast is the the difference between the thick and thin parts of a letterform’s stroke. A monolinear typeface has low stroke contrast, and is the opposite of a high-contrast face. A typical trait of a typeface with varying optical sizes is that display sizes have higher contrast than body sizes.

Serifs tend to have higher contrast strokes than sans serifs.