From WordNet (r) 3.0 (2006) :
arity
n 1: the number of arguments that a function can take
From The Free On-line Dictionary of Computing (30 December 2018) :
arity
The number of arguments a function or
operator takes. In some languages functions may have
variable arity which sometimes means their last or only
argument is actually a list of arguments.
(1997-07-21)