Illegible means impossible or very difficult to read because the writing is so messy, faded, or unclear. When your handwriting gets so sloppy that even you can't figure out what you wrote, it's illegible.

An old letter with smudged ink might be illegible after sitting in an attic for decades. A doctor's prescription can become illegible when scribbled too quickly (which is such a common problem that pharmacies sometimes have to call doctors to clarify). If you write your answers on a test so carelessly that your teacher can't make out the letters, your work might be marked wrong simply because it's illegible.

Think of it as the opposite of clear, neat writing. When something is illegible, the letters might blur together, the spacing might be terrible, or the writing might be so cramped and tangled that picking out individual words becomes impossible.

Throughout history, illegible documents have caused serious problems, from lost wills to misunderstood medical instructions.