When something is unreadable, it's impossible or extremely difficult to read. A doctor's handwriting might be so messy that the prescription becomes unreadable. A water-damaged book with blurred, running ink is unreadable. If you write in tiny letters crammed too close together, your teacher might hand back your paper and ask you to rewrite it because it's unreadable.

The word can describe physical problems with text: faded signs, smudged pencil marks, or text written in such a fancy script that no one can decode it. It can also describe digital problems, like when a file won't open on your computer or appears as jumbled nonsense characters.

In a different sense, unreadable can describe a person's face or expression when you can't tell what they're thinking or feeling. If your friend has an unreadable expression during a game of poker, you can't figure out whether they have good cards or bad ones. Their face gives nothing away.

When critics call a book unreadable, they usually mean it's so boring, confusing, or poorly written that finishing it feels impossible, even though the words themselves are physically clear on the page.