Fundamental Lemma of Calculus of Variations
If
with Continuous second Partial Derivatives, then 
on the Open Interval
.- Socrates' Paradox
- Soddy Circles
- Soddy Line
- Soddy Points
- Soddy's Hexlet
- Sofa Constant
- Soldner's Constant
- Solenoidal Field
- Sol Geometry
- Solid
- Solid Angle
- Solid Geometry
- Solid of Revolution
- Solid Partition
- Solidus
- Solitary Number
- Soliton
- Solomon's Seal Knot
- Solomon's Seal Lines
- Solomon's Seal Polygon
- Solvable Congruence
- Solvable Group
- Solvable Lie Group
- Soma Cube
- Somer-Lucas Pseudoprime