Fubini Principle
If the average number of envelopes per pigeonhole is 
, then some pigeonhole will have at least envelopes. Similarly, there must be a pigeonhole with at most envelopes.
- Curl
- Curlicue Fractal
- Curl Theorem
- Current
- Curtate Cycloid
- Curtate Cycloid Evolute
- Curvature
- Curvature Center
- Curvature Scalar
- Curvature Vector
- Curve
- Curve of Constant Breadth
- Curve of Constant Precession
- Curve of Constant Slope
- Curve of Constant Width
- Curvilinear Coordinates
- Cushion
- Cusp
- Cusp Catastrophe
- Cusp Form
- Cusp Map
- Cusp Point
- Cutting
- Cut-Vertex
- CW-Approximation Theorem