Identity and access
At an event, staff check your ID to confirm who you are: authentication. They check your badge to see which areas you may enter: authorisation.
In a digital product, signing in with a password or passkey checks your identity. The product then decides whether you may view a private document or change an account setting. Even with single sign-on, each product decides what you can access.
Both checks matter
Signing in does not grant access to everything. The service must check permissions before sharing private data or making a change; hiding a button is not enough. Developers sometimes shorten authentication to AuthN and authorisation to AuthZ.