“Halftone: In photoengraving, a process in which gradations of light (value) are obtained by manipulating the density of minute dots on the printing surface. The conversion of image to dots is achieved by photographing the subject through a special screen. Halftones have been in use since 1878.”
Excerpted from: Diamond, David G. The Bulfinch Pocket Dictionary of Art Terms. Boston: Little Brown, 1992.