Pasch's Axiom
In the plane, if a line intersects one side of a Triangle and misses the three Vertices, thenit must intersect one of the other two sides. This is a special case of the generalized Menelaus' Theorem with 
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- Involute
- Involution (Group)
- Involution (Line)
- Involution (Operator)
- Involution (Set)
- Involution (Transformation)
- Irradiation Illusion
- Irrationality Measure
- Irrationality Sequence
- Irrational Number
- Irreducible Matrix
- Irreducible Polynomial
- Irreducible Representation
- Irreducible Semiperfect Number
- Irreducible Tensor
- Irredundant Ramsey Number
- Irreflexive
- Irregular Pair
- Irregular Prime
- Irregular Singularity
- Irrotational Field
- Isarithm
- ISBN
- I-Signature
- Island