Arity is the number of arguments that a relation can take.
If a relation can take an arbitrary number of arguments, its arity is undefined. The arity of a function is one more than the number of arguments it can take, in keeping with the unified treatment of functions and relations. KIF 2.2 doesn't require one to declare the arity of a relation, nor does it require one to use a relation with a consistent number of arguments. However, relations defined with Ontolingua are always of fixed arity, which Ontolingua asserts as part of the translation. This is to facilitate sharing over implemented frame systems, most of which do not support variable-arity relations.