The smallest infinite cardinal number. The cardinality of any set which can be put in one-to-one correspondence with the set of natural numbers. Such sets are said to be countable or denumerable. One of the apparent paradoxes in number theory is that the set of rational numbers between 0 and 1, the set of all rational numbers, and the set of natural numbers all have the same cardinality. The symbol ℵ0 is used.