unreadable

1. A document, book, or paper which cannot be read because of damage, smearing, etc.The ink had become so faded that the document was unreadable
2. Areas of a document or other paper which have been blacked out or redacted in order to prevent others from reading that portion of the material.The document he wrote told about several murders that had been committed; to protect his identity, his name and address were made unreadable by redacting them over with black marker.
3. Some written material which, because of poor quality, is unusable as reading material.He wrote so poorly in cursive that his letters are considered unreadable
4. Some written material which, because of inadequate plot, character development, story or other defects is not capable of holding an audience.Her writing is considered by some to be a new type of story; others do not like it and consider it unreadable.
5. Electronic media which the signals or data encoded upon it can no longer be interpreted due to degradation or damageWe have over 500 3 1/2" floppy diskettes, however, most of them are unreadable because they became demagnetized
6. Electronic media which the signals or data encoded upon it can no longer be interpreted due to the media being of such an age that equipment to read the media is no longer availableWe have some 300 5 1/4" diskettes that are unreadable because we no longer have any computers with that size disk drive.

In cases 1 and 2, as mentioned above by rfc1394, I would beg to differ. The proper term in these cases would not be unreadable, but rather illegible, which is used when something is impossible to read because of actual physical conditions, such as handwriting or the condition of the material written upon.

I am unsure of whether this should also be applied to cases 5 and 6, but I think that the common usage leans more toward unreadable.