Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 noun The number of arguments or operands taken by a function or operator.
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 noun logic, mathematics, computer science The number of
arguments oroperands afunction oroperation takes. For arelation , the number ofdomains in thecorresponding Cartesian product .
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 noun the number of arguments that a function can take
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
from Wiktionary, Creative Commons Attribution/ShareAlike License
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Examples

r is called the arity of the function f, i.e., the number of arguments that it takes.
Computability and Complexity Immerman, Neil 2008

Then due to its popularity, the dates were extended twice until the end of August.
Archive 20090601 Hels 2009

A structure contains interpretations of certain predicate, function and constant symbols; each predicate or function symbol has a fixed arity.
Model Theory Hodges, Wilfrid 2009

A signature is a set of individual constants, predicate symbols and function symbols; each of the predicate symbols and function symbols has an arity
Firstorder Model Theory Hodges, Wilfrid 2009

In 1870 Peirce published a long paper “Description of a Notation for the Logic of Relatives” in which he introduced for the first time in history, two years before Frege's Begriffschrift a complete syntax for the logic of relations of arbitrary adicity (or: arity).
Nobody Knows Nothing 2009

Then due to its popularity, the dates were extended twice until the end of August.

Again: rheme (by which Peirce meant a relation of arbitrary adicity or arity) was a first, proposition was a second, and argument was a third.
Nobody Knows Nothing 2009

These axioms tacitly specify the arity of a combinator as well as their reduction (or contraction) pattern.
Combinatory Logic Bimbó, Katalin 2008

P and Q (or for predicates of higher arity when the variable in their last argument is bound).
Combinatory Logic Bimbó, Katalin 2008

Predicates have a fixed finite arity in FOL, and nothing precludes binding at once a variable in the first argument of one predicate and in the second argument of another predicate.
Combinatory Logic Bimbó, Katalin 2008

In general, the arity of an expression is equal to the "number of things" matched by that expression.
Ohm: Parsing Made Easy Patrick Dubroy, coauthor of Ohm 2023
sionnach commented on the word arity
the number of arguments that a predicate takes. Generally, a predicate with arity n is called an nplace predicate. Another term for arity is adicity.
October 14, 2009