What is Single Sign-On (SSO)?
Authenticating once with a central identity provider to access many applications without separate logins for each.
Definition
SSO lets a user authenticate once against an identity provider (IdP) — Okta, Microsoft Entra ID, Google Workspace, Keycloak — and then access many applications without re-entering credentials. Each application (the service provider) trusts assertions from the IdP, exchanged via SAML 2.0 in the enterprise world or OpenID Connect, the identity layer on OAuth 2.0, in modern apps. Beyond convenience, SSO centralizes control: enforced MFA, session policies, and instant deprovisioning when an employee leaves, replacing a scramble across dozens of apps. SCIM often accompanies it to sync user accounts automatically. The IdP becomes critical infrastructure and a high-value target.