A measure of the number of tonal levels, or densities, in a digital image (which might be an original digital or scanned photograph, so it's relevant to screen printing). An image formed of a range of such tones is called a "Greyscale".The term "grey levels" is also used to refer to coloured images, in the specific sense of describing the tones within each colour channel or separation.
Greyscale printheads are able to vary the density of individually printed dots, contributing to the tonal variation of the final image. See Printhead.
Pronounced 'gooey'. Stands for Graphical User Interface, meaning the combination of menus, icons and mouse point-and-click method of controlling computer programs.