A thumbprint is the unique pattern of curved lines and ridges on the pad of your thumb. If you press your thumb onto an ink pad and then onto paper, you'll see this pattern clearly: a swirl of lines that belongs only to you. No two people have the same thumbprint.

This uniqueness makes thumbprints incredibly useful for identification. Police detectives dust crime scenes for fingerprints (including thumbprints) because they can prove who touched something. Many phones and computers now use thumbprint scanners instead of passwords: you press your thumb to the sensor and it recognizes your specific pattern. Some schools use thumbprint scanners in their cafeterias so students can pay for lunch without carrying cash or cards.

The word can also describe the mark itself that your thumb leaves behind. You might leave a thumbprint on a window you just cleaned, or a baker might leave a thumbprint in the center of a cookie before adding jam (making what are called thumbprint cookies).

Beyond the physical meaning, people sometimes say something has someone's thumbprint on it when it shows that person's distinctive style or influence, like how you might recognize your friend's drawing by their particular way of sketching faces.