A countable set is a set that is either finite or denumerable. However, some authors (e.g., Ciesielski 1997, p. 64) use the definition "equipollent to the finite ordinals," commonly used to define a denumerable set, to define a countable set.
Countable Set
See also
Aleph-0, Aleph-1, Countably Infinite, Denumerable Set, Finite, Infinite, Uncountably InfiniteExplore with Wolfram|Alpha
References
Ciesielski, K. Set Theory for the Working Mathematician. Cambridge, England: Cambridge University Press, 1997.Croft, H. T.; Falconer, K. J.; and Guy, R. K. Unsolved Problems in Geometry. New York: Springer-Verlag, p. 2, 1991.Referenced on Wolfram|Alpha
Countable SetCite this as:
Weisstein, Eric W. "Countable Set." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CountableSet.html