Acute Angle
An Angle of less than 
Radians (90°) is called an acute angle.
- Undecagon
- Undecidable
- Undecillion
- Undetermined Coefficients Method
- Undulating Number
- Unduloid
- Unexpected Hanging Paradox
- Unfinished Game
- Unhappy Number
- Unicursal Circuit
- Uniform Apodization Function
- Uniform Boundedness Principle
- Uniform Convergence
- Uniform Distribution
- Uniform Polyhedron
- Uniform Variate
- Unimodal Distribution
- Unimodal Sequence
- Unimodular Group
- Unimodular Matrix
- Unimodular Transformation
- Union
- Uniplanar Double Point
- Unipotent
- Unique