Indecipherable means impossible or extremely difficult to read, understand, or figure out. When a doctor's handwriting is so messy that even a pharmacist can't read the prescription, it's indecipherable. When an ancient stone tablet has worn away so much that scholars can't make out the symbols, the text has become indecipherable.

You might encounter indecipherable handwriting in old letters, indecipherable audio in a scratchy recording, or indecipherable instructions that make no sense no matter how many times you read them. Sometimes a classmate's notes are so rushed and sloppy that they're indecipherable to everyone except the person who wrote them.

The word often suggests frustration: you want to understand something, but it's just too unclear, too damaged, or too poorly written. When something is merely difficult to read, we might say it's “hard to decipher,” but when it crosses the line into truly impossible, it becomes indecipherable.