The distance at which a person will begin to appear unattractive to another person looking at them. Outside that distance the person in question will appear attractive. Note that this varies wildly with personal preference and/or alchohol consumption. Can also vary based on certain body part characteristics, which can result in "ugly radius asymmetry". There is enough variability within the human population for ugly radii to exist from zero to infinity.
From 20 feet away, Stephanie looks mighty hot, but her ugly radius is about three feet. With makeup and a liitle alcohol in me it's reduced to a foot or less.