Wiktionary
n. (context logic mathematics computer science English) The number of arguments or operands a function or operation takes. For a relation, the number of domains in the corresponding Cartesian product.
Wikipedia
In logic, mathematics, and computer science, the arity of a function or operation is the number of arguments or operands that the function takes. The arity of a relation (or predicate) is the dimension of the domain in the corresponding Cartesian product. (A function of arity n thus has arity n+1 considered as a relation.) The term springs from words like unary, binary, ternary, etc. Unary functions or predicates may be also called "monadic"; similarly, binary functions may be called "dyadic".
In mathematics arity may also be named rank, but this word can have many other meanings in mathematics. In logic and philosophy, arity is also called adicity and degree. In linguistics, arity is usually named valency.
In computer programming, there is often a syntactical distinction between operators and functions; syntactical operators usually have arity 0, 1, or 2 (the ternary operator ?: is also common). Functions vary widely in the number of arguments, though large numbers can become unwieldy. Some programming languages also offer support for variadic functions, i.e., functions syntactically accepting a variable number of arguments.
Usage examples of "arity".
It was rather comforting to feel the easy familiarity of her companionship with Ivor Davidson, her mentor and practically her foster-father.
That sort of thing had happened a few times before, especially when she was tired, but never with such clarity and distinction.
The smell of the old wood pleased her with a faint familiarity, as with the spices in the stew last night.
Darkover comforted her with its near-familiarity, but she found it disquieting as well.
For the moment, it was the best she could do, and having sorted out the matter as well as she could, Margaret had a sense of clarity.
Lew had assured her that the room was built long after Ashara was dead, so she guessed she had picked up the sense of familiarity from him, not from the long-gone Keeper.
Always he stressed the power of our surface races, their multitudes, their solidarity.
He paced within the perimeters of the fence, muttering curses and vulgarities that made her ears burn.
Why do I get the impression that you're not thinking as much about the charity you're supporting as you are of getting your picture in the papers?
Each line was a glowing account of her charity work, though under the circumstances it read like an accusation.
Her hair was sleeked back, which may have added to the pronounced angularity of her face.
With a familiarity Borowitz's juniors could hardly credit, he then said: 'Gregor, please do not ask it.
Also, he didn't wear the wide-eyed, wary/innocent look of the tourist but a self-satisfied air of familiarity, of belonging.
As if his eyes were the lenses of binoculars, they seemed to focus on the place, blowing it up large and with incredible clarity and detail.
And because it is a true adage that famili arity breeds contempt, he had almost forgotten just how much evil that voice in his head contained.