Rectifiable Set
The rectifiable sets include the image of any Lipschitz Function 
from planar domains into 
. Thefull set is obtained by allowing arbitrary measurable subsets of countable unions of such images of Lipschitzfunctions as long as the total Area remains finite. Rectifiable sets have an ``approximate'' tangent plane at almost every point.
References
Morgan, F. ``What is a Surface?'' Amer. Math. Monthly 103, 369-376, 1996.
- Stieltjes' Theorem
- Stieltjes-Wigert Polynomial
- Stirling Cycle Number
- Stirling Number of the First Kind
- Stirling Number of the Second Kind
- Stirling's Approximation
- Stirling Set Number
- Stirling's Finite Difference Formula
- Stirling's Formula
- Stirling's Series
- Stirrup Curve
- Stochastic
- Stochastic Calculus of Variations
- Stochastic Group
- Stochastic Matrix
- Stochastic Process
- Stochastic Resonance
- Stokes Phenomenon
- Stokes' Theorem
- Stolarsky Array
- Stolarsky-Harborth Constant
- Stolarsky's Inequality
- Stomachion
- Stone Space
- Stone-von Neumann Theorem