Actually, the number of degrees the nose is pulled through per second is probably the best definition, as turn radius is not nearly as important as turn rate. But I guess that wasn't clear from my post. Sorry!
Well. . .I still have to insist "corner" is the quickest-tightest turn. And when flying air-to-air, turn rate and radius are very important, indeed.
From day one in fighter lead-in to your last day in the jet, corner is defined as the speed at which your plane has the smallest turn radius and the highest turn rate--pulling your max instantaneous G at the slowest speed. Sustained Corner is something a bit less.
To quote my 'ol AT-38-B Air-to-Air Phase Manual (Aug 1983), "Corner Velocity is the minimum airspeed at which the maximum allowable aircraft G limit can be instantaneously generated. 'Quickest/Tightest Turn.'"
If you do a yahoo search on "corner velocity" you will come up with many hits from some good sites.
For example:
http://users.vei.net/taurion/ht-actprimer.html "The speed at which the optimum instantaneous turn rate of any airplane occurs is called the corner velocity. This figure is an instantaneous one (iCV), because as soon as you begin tuning at this velocity, you will bleed speed and your turn rate will slow down in direct proportion. The speed at which any airframe has the fastest sustained turn rate (sCV) is always a little slower. This is the speed at which your plane will have the highest turn rate at which you will not bleed speed at any given thrust setting."
Anyway, thanks for making recall fondly burst capillaries in my arms and the comforting feel of 9-G's.