Foil blocking
Also called Foil stamping, hot foil stamping, foiling
A metallic film (gold, silver, copper or a colour) transferred onto paper or cloth by a heated die under pressure. The film only takes where the die presses. Seen on cover titles and on the spines of hardbacks.
In practice
The die is made once, from vector artwork, so its cost does not depend on the print run and weighs most on a short one. Very fine detail fills in, and adhesion varies with the surface: cloth, soft-touch lamination. Ask for a test on the chosen material when it is anything out of the ordinary.