artwork in which two or more inks overlap.

Overprinting is commonly used for two purposes:  To create new colors by combining inks, and to eliminate registration issues.

The former is commonly seen in process color printing, in which red (for example) is created by overprinting magenta and yellow. This technique can be used with spot colors, as well, although the results are less predictable.

The latter is usually seen with black text and line art on colored backgrounds. Because black ink printed over other inks will usually still appear black, black text and line art can overprint background colors without the need for trapping.