Single sign-on (SSO) is an authentication arrangement that lets a user sign in once to a central identity provider and then reach many connected applications without entering credentials again. One morning login can open email, file shares, and cloud apps for the rest of the day.
The upside is fewer passwords to manage and a single place to enforce authentication policy, which usually improves both usability and control. The catch is concentration of risk, because if the identity provider is breached, an attacker may inherit access to everything behind it. That is why strong, phishing-resistant MFA on the SSO front door is not optional, and why session handling and token lifetimes deserve real care. Do not confuse SSO with a password manager, since one federates trust while the other simply stores separate credentials.
How is SSO tested on the CISSP?
SSO falls under Domain 5, Identity and Access Management, including the federation protocols that make it work. Know how SAML, OAuth, and OpenID Connect enable it, and understand the tradeoff between convenience and centralized risk. See Domain 5: Identity and Access Management.
NIST covers federation in SP 800-63C.