[SAI_Peregrinus]

Usernames are fixed values and are generally public. Biometrics are also fixed values and are generally only slightly less public. They're both identifiers.

Passwords can be changed and are secrets. They're authenticators.

The difference between them is exactly the difference between identifiers and authenticators. Misunderstanding this difference causes tons of issues, in a wide variety of situations. The most notable one recently is probably Social Security Numbers being used as both, which leads to identity theft.