Theorem
A statement which can be demonstrated to be true by accepted mathematical operations and arguments. In general, a theoremis an embodiment of some general principle that makes it part of a larger theory.
According to the Nobel Prize-winning physicist Richard Feynman 
(1985), any theorem, no matter howdifficult to prove in the first place, is viewed as ``Trivial'' by mathematicians once it has been proven. Therefore, there are exactly two types of mathematical objects: Trivial ones, and those which have not yet beenproven.
References
Feynman, R. P. and Leighton, R. Surely You're Joking, Mr. Feynman! New York: Bantam Books, 1985.
- Order (Vertex)
- Ordinal Number
- Ordinary Differential Equation
- Ordinary Differential Equation--First-Order
- Ordinary Differential Equation--First-Order Exact
- Ordinary Differential Equation Second-Order
- Ordinary Differential Equation--System with Constant Coefficients
- Ordinary Double Point
- Ordinary Line
- Ordinary Point
- Ordinate
- Ore Number
- Ore's Conjecture
- Ore's Theorem
- Orientable Surface
- Orientation (Plane Curve)
- Orientation-Preserving
- Orientation-Reversing
- Orientation (Vectors)
- Origami
- Origin
- Ornstein's Theorem
- Orr's Theorem
- Orthic Triangle
- Orthobicupola