The beth numbers are infinite cardinal numbers defined in a similar manner to the aleph numbers, as described below. They are written , where is beth, the second letter of the Hebrew alphabet, and is an ordinal number.

We define to be the first infinite cardinal (that is, ). For each ordinal , we define . For each limit ordinal , we define .

Note that is the cardinality of the continuum.

For any ordinal the inequality holds. The Generalized Continuum Hypothesis is equivalent to the assertion that for every ordinal .

For every limit ordinal , the cardinal is a strong limit cardinal. Every uncountable strong limit cardinal arises in this way.