Cardinality
An infinite set is considered countable if they can be listed without missing any (that is, if there is a one-to-one correspondence between it and the set of natural numbers ). Examples include the rational numbers, integers, and natural numbers. Such sets have a cardinality that we call  (pronounced "aleph null", "aleph naught" or "aleph zero"). Sets such as the real numbers are not countable, since given any finite or infinite list of real numbers, it's always possible to find a number that's not on that list. The real numbers have a cardinality of —the cardinality of the continuum.