Single sign-on (SSO) is a property of access control of multiple related, yet independent, software systems. With this property, a user logs in with a single ID and password to gain access to a connected system or accomplished using the Lightweight Directory Access Protocol (LDAP) and stored LDAP databases on (directory) servers. A simple version of single sign-on can be achieved over IP networks using cookies but only if the sites share a common DNS parent domain.

For clarity, it is best to refer to systems requiring authentication for each application but using the same credentials from a directory server as Directory Server Authentication and systems where a single authentication provides access to multiple applications by passing the authentication token seamlessly to configured applications as single sign-on.