A PIN (or personal identification number) is a numeric password used in the process of authenticating a user to a system.
In Trezor security design, the PIN plays a critical role because it protects the device against physical attacks. Trezor employs an original method of entering PIN that is both convenient and highly secure. Therefore, unlike a PIN entered on a PC or a smartphone, a PIN entered on Trezor devices cannot be compromised by a malware.
The PIN is set during the device setup and can be changed later. The PIN can be up to nine digits long and it is recommended to have a PIN that is four or more digits long without repetitions. For security reasons (see PIN brute-forcing attack), there is an increasing delay after each unsuccessful PIN entry and the device automatically wipes itself after the 15th invalid attempt. Read more about PIN use in Trezor blog.