definite article - "it is *the* exception" means one and only exception?

– You can't cast aspersions on someone just because they're wearing a cape. Superman wore a cape!

– OK, Superman is the exception.

Does the here convey the meaning that it is the only exception, and there are no other exceptions?

Answer

Technically, yes. That said, I'd caution anyone against reading too much into the definite article, or interpreting it too literally.

Fact is, in conversation at least, the article the is sometimes used to mean a notable or a prominent. For example:

Milan is the place to be if you want to be in the fashion world.

Oh, really? What about Paris?

Here's another example:

All politicians are self-centered and corrupt!
What about Mahatma Gandhi?
Oh, well, Gandhi was the exception.

If pressed, the speaker in this dialog would probably admit there are some other notable exceptions, too: Gladstone, Carter, Havel, Mandela, to name a few. Just like the word "all" was an exaggeration – a sweeping (and perhaps unfair) generalization – the word "the" isn't necessarily the most accurate article to use. But people still use it that way.

When I look up the in the dictionary, I see this definition – which is often how it's explained on ELL:

denoting one or more people or things already mentioned or assumed to be common knowledge ^{[emphasis added]}

However, I find several other meanings listed as well, such as:

• used to refer to a person, place, or thing that is unique



• used to point forward to a following qualifying or defining clause or phrase


• used with an adjective to refer to those people who are of the type described


• used to indicate that someone or something is the best known or most important of that name


• used to make a generalized reference to something rather than identifying a particular instance

That last one is the meaning that aligns with: "When I look it up in the dictionary...".

So, while I would agree that:

Okay, Superman is the exception.

suggests that Superman is the sole exception, I would not say that the word "the" proves that Superman is the sole exception.