Radius stands for Remote Authentication Dial In User Service (From http://en.wikipedia.org/wiki/RADIUS).
A RadiusServer is an WirelessSecurity tool. It is an IdentityManagement server for remote users. See http://www.tech-faq.com/radius.shtml
It is said RadiusServer is "the de facto standard authentication provider", even though it is not the 802.11i standard for WiFi.
A GnuProject version can be seen at http://savannah.gnu.org/projects/radius/