A film of ink printing in the non-image area of the plate where it should not print. Scumming as we use the term goes around the cylinder and is generally caused by too much ink or not enough water reaching the plate.
The non-image area of a printing plate attracts ink. This unwanted ink will print on a sheet and give a dirty background appearance. The problem is usually fixed by adjusting the ink/water balance on the press. See also Catch-up.
A term referring to the press plate picking up ink in the non-printing areas for a variety of reasons, basically due to spots or areas not remaining desensitized.