PAM (Pluggable Authentication Modules)
From MoodleDocs
Location: Settings link in Settings > Site administration > Plugins > Authentication > Manage authentication
With PAM you can use the same user database for different applications log ins (SSO - Single Sign On). PAM also supports different authentication processes as required.
This method uses PAM to access the native usernames on this server. You have to install PAM Authentication Module in order to use this module.