From a couple of searches. "The radius of a circle is the distance from the centre of the circle to its edge. The radius of a regular polygon is the distance f

From a couple of searches. "The radius of a circle is the distance from the centre of the circle to its edge. The radius of a regular polygon is the distance from the centre of the polygon to any one of its vertices. The plural of radius is radii." From maths.org, CAMBRIDGE UNIVERSITY. http://thesaurus.maths.org/mmkb/entry.html?action=entryByConcept&id=18 http://thesaurus.maths.org/mmkb/entry.html?action=entryByCon... From the BBC, http://www.bbc.co.uk/skillswise/tutors/lessonplans/spelling/rtf/plurals_wsheet.rtf http://www.bbc.co.uk/skillswise/tutors/lessonplans/spelling/... From the Oxford dictionary, http://www.askoxford.com:80/concise_oed/radii?view=uk http://www.askoxford.com:80/concise_oed/radii?view=uk From the Chambers dictionary, http://www.chambersharrap.co.uk/chambers/features/chref/chref.py/main?query=radii&title=21st http://www.chambersharrap.co.uk/chambers/features/chref/chre... I'm sure both are generally accepted. Geniuses, Genii, Radiuses, Radii. Bear in mind I'm talking about English, not American-English which may be different. I'm certainly not just making this stuff up though ;)