Dot gain refers to a characteristic of halftone printing whereby reflectance of the printed material is less than would be expected from the percentage area coverage of the dots set on film by the imagesetter, or the percentage area coverage expected from a digital code value in the digital representation of the image.
Dot gain is also called "tone value increase" (TVI). The term TVI is confusing because in domains other than printing - for example, in color science - value is zero at black, and increases towards white; what a printer calls "tone value increase" would be called, by a color scientist, "tone value decrease"!
Dot gain is dependent upon many factors associated with the ink, the press, and the substrate being printed.
Some often-encountered dot gain percentages include:
coated sheetfed lithography at 150 lines per inch 15%
uncoated sheetfed offset lithography at 133 lines per inch 20%
coated web offset lithography at 133 lines per inch 22%
newsprint web offset lithography at 100 lines per inch 30%