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.
- Intersection
- Interspersion
- Intersphere
- Interval
- Interval Graph
- Interval Order
- Intrinsically Linked
- Intrinsic Curvature
- Intrinsic Equation
- Invariant
- Invariant Density
- Invariant (Elliptic Function)
- Invariant Manifold
- Invariant Point
- Invariant Subgroup
- Inverse Cosecant
- Inverse Cosine
- Inverse Cotangent
- Inverse Curve
- Inverse Filter
- Inverse Function
- Inverse Hyperbolic Cosecant
- Inverse Hyperbolic Cosine
- Inverse Hyperbolic Cotangent
- Inverse Hyperbolic Functions