Fixed is a synonym for any form of erosion-proofing, and is generally used as such to refer to items when wishing for them (because the game uses only one bit for erodeproofing or fixedness, they are all internally equivalent). The term fooproof is also used, but the game does not recognize this.
Originally, fixed referred to crysknives; a fixed crysknife will revert to a worm tooth when dropped only 10% of the time.