Overprint
Printing one ink directly on top of another, so the two combine, instead of leaving a gap for it.
Also: overprinting, knockout
When two coloured shapes overlap in a design, the printer has a choice. Normally the lower colour is removed where the upper one sits, so the top colour prints on bare paper; this is called a knockout. With overprint, the top ink is laid over the lower one and the two mix, as yellow over blue gives green. Black text is nearly always overprinted, which avoids white slivers if the plates shift. Used on purpose, overprinting gives a third colour for free, a favourite device of poster artists.