OAuth / OIDC(Open Authorization / OpenID Connect)
Standards that let apps delegate authentication and authorization to a trusted identity provider.
OpenID Connect (built on OAuth 2.0) is how 'Log in with…' buttons work. Self-hosted identity providers like Authentik and Keycloak speak OIDC so your apps can share one secure login flow.
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SSO(Single Sign-On)
Logging in once with a central identity provider to access many separate applications.
2FA / MFA(Two-Factor / Multi-Factor Authentication)
Requiring more than just a password to log in — typically something you know plus something you have.