Any set which can be put in a one-to-one correspondence with the natural numbers (or
integers) so that a prescription can be given for identifying
its members one at a time is called a countably infinite (or denumerably infinite)
set. Once one countable set 
is given, any other set which can be put into a one-to-one
correspondence with 
is also countable. Countably infinite sets have cardinal
number aleph-0.
Examples of countable sets include the integers, algebraic numbers, and rational numbers. Georg Cantor showed that the number of real numbers is rigorously larger than a countably infinite set, and the postulate that this number, the so-called "continuum," is equal to aleph-1 is called the continuum hypothesis. Examples of nondenumerable sets include the real, complex, irrational, and transcendental numbers.
See also
Aleph-0, Aleph-1, Cantor Diagonal Method, Cardinal Number, Continuum, Continuum Hypothesis, Countable Set, Hilbert Hotel, Infinite, Infinity, Uncountably Infinite
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References
Courant, R. and Robbins, H. "The Denumerability of the Rational Number and the Non-Denumerability of the Continuum." ยง2.4.2 in What Is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed. Oxford, England: Oxford University Press, pp. 79-83, 1996.Jeffreys, H. and Jeffreys, B. S. Methods of Mathematical Physics, 3rd ed. Cambridge, England: Cambridge University Press, p. 10, 1988.
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Cite this as:
Weisstein, Eric W. "Countably Infinite." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/CountablyInfinite.html