Definition of arity:
(n) :
(logic, mathematics, computer science) The number of arguments or operands a function or operation takes. For a relation, the number of domains in the corresponding Cartesian product.
(n) :
(mathematics, computer science) The maximum number of child nodes that any node in a given tree (data structure) may have.
(n) :
(Lojban grammar) the number of arguments (in Lojban grammar called sumti) specified in the definition of a selbri. (the selbri combined with the sumti make up a bridi).