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Authentication answers “who are you?” through login, OAuth, magic links, passkeys, passwords, or SSO. It does not by itself answer what the user may do. That second question is authorization.
Plainly
Think of Authentication as the lock and rules that keep people and data safe. Proving who a user is.
In practice
Use it when a workflow touches secrets, permissions, user data, payments, or untrusted input. In practice, define the owner, input, output, and failure mode before you rely on it.