Coming across "unary operators" might happen in JavaScript, or (m)any other programming language(s). A couple examples for what is a "unary operator".
In JavaScript you may find: +1, -42, + "1", !true, delete x. All those are unary operators as you can also read on MDN and in the spec. But what does the "unary" really mean? And why is 1 + 1 not using a unary operator?

Definition

In Merriam-Webster "unary" is defined like so

having, consisting of, or acting on a single element, item, or component

The +, the -, the ! or the delete are applied to one "element". In spec terms, a unary "operator" is applied to one and only one variable, identifier or alike. In +1 the operator is the + and it is applied on the 1. Because it is applied only to one operand, in this case the 1, it is a unary (not binary, not ternary, not quaternary) operator.

What is the Opposite of "Unary"?

Just before I used "binary", "ternary" and "quaternary", because I jumped a little bit ahead. I had asked myself what else "unary" does belong to, if there was something like "multi-nary" or alike. So I hunted the web and found the answer on english.stackexchange.com. This seems to be the place where people who know language(s) seem to hang out.

On that site exactly my question was asked and discussed "Is there a word like “unary” meaning “consisting of more than one element”?". The discussion of the term is short and insightful (not so full of trolling as the web sometimes offers). And we end up in computer science space again, the final answer that leads me to the definition of "Arity" resolves all the mystery and clears up the space around the "??-ary" word.

You can use multiple-arity if different numbers of arguments are accepted (e.g. a polymorphic function/method), or n-ary if you want to be indefinite about the arity.

Arity

The above is the best answer on what the opposite or "unary" is, it also links on to the wikipedia article on "Arity" where one can go into depth and sees all the other arities, as touched on above. The variable number of "elements", or arguments is called "n-ary" as the according paragraph explains.

Back to JavaScript

Coming back to JavaScript. When reading about the "unary plus" on MDN it is easier to understand now. The difference to the other "+", the arithmetic operator is not just the name but also the functionality. The one adds two "things" (the arithmetic operator) and the other only converts one "thing" (the unary operator) to a Number type. This answers the second question I started with, about the difference to 1 + 1.

Let's see three unary and one arithmetic operator in action:

> + '1'
1

> 1 + 1
2

> typeof + '1'
"number"

Are you seeing the three unary operators? Hint: typeof is also a unary operator.

The ECMAScript Spec

Not very surprisingly the spec also has the chapter "Unary + Operator". Understanding that this is not a JavaScript specific term, but seemingly even more computer science, even rooted in math takes away more of the fear of touching it, because the usefulness and possible application of the term has just increased.

So far my learnings on "unary". If you find any errors or possible improvements let me know. I am always curious.