The tendency for paper to be distorted and not lay flat when it encounters large changes in heat and humidity. Curl occurs either as roll set or structural curl. Roll set curl, which occurs across the grain of the sheet, is created as the result of the paper being formed and wrapped around a roll core in a circular production process.
In paper, the distortion of a sheet due to differences in structure or coatings from one side to the other, or the absorption of moisture on an offset press. Not lying flat and tending to form into cylindrical or wavy shapes.