Absolute Geometry
Geometry which depends only on the first four of Euclid's Postulates and not on the Parallel Postulate. Euclid 
himself used only the first four postulates for the first 28 propositions of the Elements">Elements, butwas forced to invoke the Parallel Postulate on the 29th.
References
Hofstadter, D. R. Gödel, Escher, Bach: An Eternal Golden Braid. New York: Vintage Books, pp. 90-91, 1989.
- Class Number Formula
- Class Representative
- Class (Set)
- Clausen Formula
- Clausen Function
- Clausen's Integral
- CLEAN Algorithm
- CLEAN Beam
- CLEAN Map
- Clebsch-Aronhold Notation
- Clebsch Diagonal Cubic
- Clebsch-Gordan Coefficient
- Clement Matrix
- Clenshaw Recurrence Formula
- Clifford Algebra
- Clifford's Circle Theorem
- Clifford's Curve Theorem
- Cliff Random Number Generator
- Clique
- Clique Number
- Clock Solitaire
- Closed Curve
- Closed Curve Problem
- Closed Disk
- Closed Form