Arity (from Latin) is the term used to refer to the number of arguments or operands in a function or operation, respectively. You’re most likely to come across this word in the realm of JavaScript when it’s used to mention the number of arguments expected by a JavaScript function.

There’s even a property named arity, of the Function object that returns the number of expected arguments in a function. It’s now obsolete though and replaced by length.

As an example, the following function has an arity of 3.

function getName(first, middle, last) {
  return first + ' ' + middle + ' ' + last;