Saint Petersburg Paradox

Salinon

Sandglass

Satanic Square

Sator Square

Schlegel Diagrams

Schoolgirls

Schroeder Stairway

Scrambled Numbers

Secant Number

Segment

Selection

Semidiabolic Square

Semigeometric Square

Semimagic Knight

Semimagic Square
Semiperfect Number

Semiprime Number

Semiregular Polyhedron

Semiregular Tessellation

Senary Order

Sequence (1)

Sequence (2)

Settlement

Seven Sevens

Sharing

Shed

Shepherd's Principle

Shoemaker's Knife

Shogi

Shopping

Shunting

Sickle of Archimedes

Sierpinski Curve

Sierpinski Triangle

Sieve

Sieve of Eratosthenes

Silhouette

Sim

Simple Jump

Simple Magic Square

Simplification

Simulation

Singleton Calendar

Six Frogs

Six Men's Morris

Skeleton Division

Skeleton Multiplication

Sliding

Sliding Small Board

Small Board

Small Plank

Smith Number

Snakes and Ladders

Snail

Snowflake

Snub Cube

Snub Dodecahedron

Sociable Number

Society Rule

Solid

Solid Flexagon

Solid Number

Solitaire

Solomon's Seal

Solution

Soma Cube

Sophism

Soustracted Square

Soustracted Triangle

Spatial Tic-Tac-Toe

Spatial Piece

Speed

Sphere

Sphinx

Spider and Fly

Spiral

Sprout

Square

Square Centered Number

Square Chessboard

Square Domino

Square Gnomonic Number

Square Grid (1)

Square Grid (2)

Square Hyperpyramidal Number 

Square Mosaic

Square Number

Square Numbers

Square Pyramidal Number

Square Squaring

Squared Rectangle (1)

Squared Rectangle (2)

Squared Square

Squaring

Staircase

Stake

Star

Star Heptagon

Star Hexagon

Star Number

Star Octogon

Star of David

Star Pentagon

Star Polygon

Star Polyhedron

Statement

Steiner Triple System

Stellated Polyhedron

Stereogram

Stick

Stifel Triangle

Stomachion

Strategic Hit

Strategy

Strict Determination

String

Submarine Battle

Substitution

Successive Numbers

Successor

Sudoku

Summand

Super Queen

Superabundant Number

Superdomino

Superprime Leader Number

Superprime Number

Surface Partition

Surfaked Square

Svastika

Sweet Day

Sword

Symmetric Elements

Symmetric Lattices

Symmetric Latin Square

Symmetric Magic Square

Syracuse Problem

Systematical Research