Given a function(in the mathematical sense) f, the arity of f is the number of arguments/inputs it takes, e.g. the absolute value function, which takes a complex number, and returns its distance on the complex plane from 0, has an arity of 1. Functions with aritys above 2 are rare in mathematics, and aritys above 3 are rare in theoretical computer science. The word arity is rarely used, instead most people combine the Latin prefix with -ary, e.g. a binary operators take 2 inputs.