Definition
Single Sign-On (SSO) is an authentication pattern where a user authenticates once with an identity provider and gains access to multiple downstream applications without re-entering credentials.
In more depth
SSO is implemented over a federation protocol — SAML 2.0 for enterprise B2B, OIDC for modern web + mobile. The IdP holds the authoritative session; downstream apps trust signed assertions from the IdP.
For B2B SaaS, "SSO" usually means "the customer's IdP signs in their users to my product." For workforce, "SSO" means "our IdP signs in our employees across all our SaaS apps." Both patterns underlie modern enterprise identity.
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