LDAP, Active Directory, OAuth, OAuth2, OpenID, OpenID Connect...... None of above are SSO conceptually.
A property of access control of multiple related, but independent software systems
http://www.tml.tkk.fi/Opinnot/Tik-110.501/1997/single_sign-on.html
http://www.tml.tkk.fi/Opinnot/Tik-110.501/1998/papers/3singlesignon/singlesign-on.htm
https://en.wikipedia.org/wiki/List_of_single_sign-on_implementations
...... 50+
https://www.apereo.org/
https://wiki.jasig.org/display/CAS/CAS+Threat+Modeling
https://github.com/dlindahl/omniauth-cas
https://github.com/intridea/omniauth-ldap
http://www.rubydoc.info/gems/ruby-net-ldap