Schwarz-Pick Lemma
If 
is an analytic map of the Disk 
into and preserves the hyperbolic distancebetween any two points, then is a disk map and preserves all distance.
References
Busemann, H. The Geometry of Geodesics. New York: Academic Press, p. 41, 1955.
- Morera's Theorem
- Morgado Identity
- Morgan-Voyce Polynomial
- Morley Centers
- Morley's Formula
- Morley's Theorem
- Morley's Triangle
- Morphism
- Morrie's Law
- Morse Inequalities
- Morse Theory
- Morse-Thue Sequence
- Mortal
- Mortality Problem
- Morton-Franks-Williams Inequality
- Mosaic
- Moser
- Moser's Circle Problem
- Moss's Egg
- Motzkin Number
- Moufang Plane
- Mousetrap
- Mouth
- Moving Average
- Moving Ladder Constant