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A function or operator can thus be described as unary, binary, ternary,etc. Terms such as 7-ary or n-ary are also used.

Sometimes, it is useful to consider a constant as an operator or function of arity 0, and hence call it nullary (or sometimes anary). For example, one formulation of the axiom of choice claims that for every non-empty set, there exists a nullary function that can be used to pick a value from the set.