Bankoff Circle
and 
in the Arbelos figure, there is a third circle 
congruent to these two as illustrated in the above figure.See also Arbelos
References
Bankoff, L. ``Are the Twin Circles of Archimedes Really Twins?'' Math. Mag. 47, 214-218, 1974. Gardner, M. ``Mathematical Games: The Diverse Pleasures of Circles that Are Tangent to One Another.'' Sci. Amer. 240, 18-28, Jan. 1979.
- Radial Curve
- Radial Point
- Radian
- Radiant Point
- Radical
- Radical Axis
- Radical Center
- Radical Integer
- Radical Line
- Radicand
- Radius
- Radius (Graph)
- Radius of Convergence
- Radius of Curvature
- Radius of Gyration
- Radius of Torsion
- Radius Vector
- Radix
- Radon-Nikodym Theorem
- Radon Transform
- Radon Transform--Cylinder
- Radon Transform--Delta Function
- Radon Transform--Gaussian
- Radon Transform--Square
- Rado's Sigma Function