A machinist’s term that refers to grinding down the surface of a piece of metal to remove rust and other imperfections.  In knife terminology cleaning or “cleaned” refers to re-grinding the surfaces of a blade to remove traces of corrosion and/or improper sharpening.  Cleaning removes all the original factory surface, any etching, and sometimes even the original contours of the blade, and substantially de-values the knife for serious collectors.  See: Mint.