Math. in the theory of sets, the smallest infinite cardinal number; the cardinal number of the set of all positive integers: symbol, ?
The first of the transfinite cardinal numbers. It corresponds to the number of elements in the set of positive integers. Also called aleph-zero.
- (set theory) The first of the transfinite cardinal numbers; corresponds to the number of positive integers, also called natural numbers. Georg Cantor showed that even all the rational numbers could be put in one-to-one correspondence with them, and are therefore countable, enumerable or denumerable.