Baseball
The numbers 3 and 4 appear prominently in the game of baseball. There are 
innings in a game, and three strikesare an out. However, 4 balls are needed for a walk. The number of bases can either be regarded as 3 (excluding HomePlate) or 4 (including it).
- Tutte's Theorem
- Twin Peaks
- Twin Prime Conjecture
- Twin Primes
- Twin Primes Constant
- Twins
- Twirl
- Twist
- Twisted Chevalley Groups
- Twisted Conic
- Twisted Sphere
- Twist Map
- Twist Move
- Twist Number
- Twist-Spun Knot
- Two
- Two-Ears Theorem
- Two-Point Distance
- Two Triangle Theorem
- Tychonof Compactness Theorem
- Type
- Type I Error
- Type II Error
- Ulam Map
- Ulam Number