Denumerable Set
A Set is denumerable if a prescription can be given for identifying its members one at a time. Such a set is saidto have Cardinal Number Aleph-0. Examples of denumerable sets include AlgebraicNumbers, Integers, and Rational Numbers. Once onedenumerable set 
is given, any other set which can be put into a One-to-One correspondence with is alsodenumerable. Examples of nondenumerable sets include the Real, Complex,Irrational, and Transcendental Numbers.
References
Courant, R. and Robbins, H. ``The Denumerability of the Rational Number and the Non-Denumerability of the Continuum.'' §2.4.2 in What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed. Oxford, England: Oxford University Press, pp. 79-83, 1996.
- Reduced Fraction
- Reduced Latitude
- Reducible Crossing
- Reducible Matrix
- Reducible Representation
- Reduction of Order
- Reduction Theorem
- Redundancy
- Reeb Foliation
- Reef Knot
- Re-Entrant Circuit
- Refinement
- Reflection
- Reflection Property
- Reflection Relation
- Reflexible
- Reflexible Map
- Reflexive Closure
- Reflexive Graph
- Reflexive Reduction
- Reflexive Relation
- Reflexivity
- Region
- Regression
- Regression Coefficient