Although "dirt" is a vague, general term in everyday use (or even in printing terminology), in papermaking it refers specifically to any foreign matter that embeds itself in the surface of the paper and which differs in color from the rest of the paper (it is usually black). Dirt may or may not be able to be lifted from the surface of the paper by a press blanket. Dirt can have deleterious results when occurring in OCR paper, designed for Optical Character Recognition machines.
To gauge how "dirty" a paper is, a "dirt count" can be calculated, providing a measure of the total black (or "dirty") area in square millimeters per square meter.