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.
- Simpson's 3/8 Rule
- Simpson's Paradox
- Simpson's Rule
- Simson Line
- Sinc Function
- Sinclair's Soap Film Problem
- Sine
- Sine-Gordon Equation
- Sine Integral
- Sines Law
- Sine-Tangent Theorem
- Singly Even Number
- Singular Homology
- Singularity
- Singular Point (Algebraic Curve)
- Singular Point (Differential Equation)
- Singular Point (Function)
- Singular Series
- Singular System
- Singular Value
- Singular Value Decomposition
- Sinh
- Sink (Directed Graph)
- Sink (Map)
- Sinusoidal Projection