backfile

As a noun backfile

is a file of documents in paper or fiche format that have no digital equivalent.

As an adjective undefined is

lacking a definition or value.

As nouns the difference between backfire and backfile

is that backfire is (firefighting) while backfile is a file of documents in paper or fiche format that have no digital equivalent.

As a verb backfire

is (of a gun or cannon) to fire in the opposite direction, for example due to an obstruction in the barrel.

As nouns the difference between backfill and backfile

is that backfill is the material that has been used to refill an excavation while backfile is a file of documents in paper or fiche format that have no digital equivalent.

As a verb backfill

is to refill a hole with the material dug out of it.