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.
- Proper Divisor
- Proper Fraction
- Proper Integral
- Proper k-Coloring
- Proper Subset
- Proper Superset
- Proportional
- Proposition
- Propositional Calculus
- Prosthaphaeresis Formulas
- Proth's Theorem
- Protractor
- Prouhet's Problem
- Prussian Hat
- Prüfer Ring
- p-Series
- Pseudoanalytic Function
- Pseudocrosscap
- Pseudocylindrical Projection
- Pseudogroup
- Pseudolemniscate Case
- Pseudoperfect Number
- Pseudoprime
- Pseudorandom Number
- Pseudorhombicuboctahedron