Straight Angle
An Angle of 
Radians.
- Rouché's Theorem
- Roulette
- Round
- Rounding
- Routh-Hurwitz Theorem
- Routh's Theorem
- RSA Encryption
- RSA Number
- Rubber-Sheet Geometry
- Rubik's Clock
- Rubik's Cube
- Rudin-Shapiro Sequence
- Rudvalis Group
- Rule
- Ruled Surface
- Rule of 72
- Ruler
- Ruler Function
- Ruling
- Run
- Runge-Kutta Method
- Runge-Walsh Theorem
- Running Knot
- Russell's Antinomy
- Russell's Paradox