Gloss Differential
A visible difference in surface sheen between inked and uninked areas of a glossy print.
In depth
Where ink covers the paper the surface reflects differently from where it does not, so an unprinted white border or a bright highlight can appear as a distinct patch of different sheen. Printers address it with a clear gloss optimiser laid over the whole sheet, and the problem does not arise on matte media.
Examples in practice
A high-key portrait on glossy paper shows the white background as a visibly different sheen from the inked areas.