A hardware sensor that can quickly read a person's unique pattern of ridges on their fingertip, to verify that person's identity. This is a type of biometric security.
It can be used in place of a password to unlock a phone, unlock certain content or services, or confirm a financial transaction.
The most common type of sensor is a small fingertip-size pad that reads the whole fingerprint at once upon being touched.
Some sensors are thinner strips that read only part of a fingertip with a single touch, but enough to confirm the fingerprint with confidence. These can be integrated into the lock button on the side of a phone, for example.
There are technologies that allow a fingerprint reader to be integrated into the display of a phone. Some of these read the fingerprint optically, while others use ultrasonic technology.
Some older sensors consisted of a thin strip that the user swiped their finger over.