Dedekind Cut
A set partition of the Rational Numbers into two nonempty subsets 
and 
such that allmembers of are less than those of and such that has no greatest member. Real Numbers can be defined using either Dedekind cuts or Cauchy Sequences.
References
Courant, R. and Robbins, H. ``Alternative Methods of Defining Irrational Numbers. Dedekind Cuts.'' §2.2.6 in What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed. Oxford, England: Oxford University Press, pp. 71-72, 1996.
- Pasch's Axiom
- Pasch's Theorem
- Pass Equivalent
- Pass Move
- Patch
- Path
- Path Graph
- Path Integral
- Pathwise-Connected
- Patriarchal Cross
- Pauli Matrices
- Pauli Spin Matrices
- Payoff Matrix
- P-Circle
- Peacock's Tail
- Peano Arithmetic
- Peano Curve
- Peano-Gosper Curve
- Peano's Axioms
- Peano Surface
- Pear Curve
- Pearls of Sluze
- Pear-Shaped Curve
- Pearson-Cunningham Function
- Pearson Kurtosis