Explain the term "Single Logout" in SSO.

Single Logout (SLO) refers to a feature in Single Sign-On (SSO) systems that ensures a user is automatically logged out from all applications or services they accessed during a session when they log out from one application. This is critical for maintaining security and user session integrity, as it prevents unauthorized access to applications once the user has chosen to end their session.

When SLO is implemented, if a user logs out from one application, the SSO mechanism communicates with all other connected applications, effectively terminating the session across all of them. This is particularly important in environments where multiple applications are accessed in a unified manner, as it enhances security by reducing the risk of leftover sessions remaining active.

The other options focus on different aspects that do not accurately describe the SLO functionality. For instance, allowing multiple users to log in at once doesn’t capture the essence of what SLO does; tracking history of user logins is unrelated to the logout process; and setting a time limit for user sessions pertains to session management rather than the logout process itself. Therefore, the mechanism of ensuring a cohesive logout across all applications in a session is captured accurately by the definition provided in the correct answer.