Authentication
In plain terms
Verifying who a user or system actually is.
Going deeper
Authentication confirms identity through passwords, tokens, biometrics, or keys. It answers 'are you who you claim to be?' and is the gate before authorization decides what you're allowed to do.
Under the hood
Factors are something you know/have/are; modern systems favor passwordless, MFA, and token-based flows over raw passwords. Distinct from authorization.
Examples
Passwords, OAuth/OIDC login, biometrics, passkeys, API keys, SSO.
Think of it like...
Like showing your passport at the airport: proving you are genuinely the person named on the ticket.