Traveling Salesman Problem
A problem in Graph Theory requiring the most efficient (i.e., least total distance) Tour (i.e., closed path)a salesman can take through each of 
cities. No general method of solution is known, and the problem isNP-Hard.
References
Platzman, L. K. and Bartholdi, J. J. ``Spacefilling Curves and the Planar Travelling Salesman Problem.'' J. Assoc. Comput. Mach. 46, 719-737, 1989.
- Skew Polyomino
- Skew Quadrilateral
- Skew Symmetric Matrix
- Sklar's Theorem
- Skolem-Mahler-Lerch Theorem
- Skolem Paradox
- Skolem Sequence
- Slant Height
- Slice Knot
- Slide Move
- Slide Rule
- Slightly Defective Number
- Slightly Excessive Number
- Slip Knot
- Slope
- Slothouber-Graatsma Puzzle
- Slutzky-Yule Effect
- Sluze Pearls
- Smale-Hirsch Theorem
- Smale Horseshoe Map
- Small Cubicuboctahedron
- Small Ditrigonal Dodecacronic Hexecontahedron
- Small Ditrigonal Dodecicosidodecahedron
- Small Ditrigonal Icosidodecahedron
- Small Dodecacronic Hexecontahedron