In logic, mathematics, and computer science, the arity (synonyms include type, adicity, and rank) of a function or operation is the number of arguments or operands that the function takes. The arity of a relation is the number of domains in the corresponding Cartesian product. The term springs from such words as unary, binary, ternary, etc.

The term "arity" is primarily used with reference to operations. If f is the function f : Sⁿ â†’ S, where S is some set, then f is an operation and n is its arity.

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