Löwenheim-Skolem Theorem
A fundamental result in Model Theory which states that if a countable theory has a model, then it has a countable model. Furthermore, it has a model of every Cardinality greater than or equal to 
(Aleph-0). This theoremestablished the existence of ``nonstandard'' models of arithmetic.
References
Chang, C. C. and Keisler, H. J. Model Theory, 3rd enl. ed. New York: Elsevier, 1990.
- Tethered Bull Problem
- Tetrabolo
- Tetrachoric Function
- Tetracontagon
- Tetracuspid
- Tetrad
- Tetradecagon
- Tetradecahedron
- Tetradic
- Tetradyakis Hexahedron
- Tetraflexagon
- Tetragon
- Tetrahedral Coordinates
- Tetrahedral Graph
- Tetrahedral Group
- Tetrahedral Number
- Tetrahedral Surface
- Tetrahedroid
- Tetrahedron
- Tetrahedron 10-Compound
- Tetrahedron 5-Compound
- Tetrahedron Inscribing
- Tetrahemihexacron
- Tetrahemihexahedron
- Tetrakaidecagon