Idempotent
An Operator 
such that 
or an element of an Algebra 
such that 
.
- Harmonic Progression
- Harmonic Range
- Harmonic Ratio
- Harmonic Segment
- Harmonic Series
- Harmonic System of Points
- Harmonious Graph
- Harnack's Inequality
- Harnack's Theorems
- Harshad Number
- Hartley Transform
- Hart's Inversor
- Hart's Theorem
- HashLife
- Hasse-Davenport Relation
- Hasse Diagram
- Hasse-Minkowski Theorem
- Hasse Principle
- Hasse's Algorithm
- Hasse's Conjecture
- Hasse's Resolution Modulus Theorem
- Hat
- Haupt-Exponent
- Hausdorff Axioms
- Hausdorff-Besicovitch Dimension