Cardinal number

Size of a possibly infinite set

In mathematics, a cardinal number, or cardinal for short, is a kind of number that measures the cardinality of a set, i.e., how many elements there are in a set. The cardinal number associated with a set ⁠ A {\displaystyle A} ⁠ is generally denoted by ⁠ | A | {\displaystyle \vert A\vert } ⁠, with a vertical bar on each side, though it may also be denoted by A {\displaystyle A} , card ⁡ ( A) , {\displaystyle \operatorname {card} (A),} or # A . {\displaystyle \#A.} Cardinality is defined in terms of bijective functions.

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