Guilloché refers to an engraving technique in which a very precise intricate repetitive patterns or design is mechanically etched into an underlying material with very fine detail. Specifically, it involves a technique of engine turning, called guilloché in French after the French engineer Guillot, who invented a machine that could scratch fine patterns and designs on metallic surfaces. This improved upon the more time-consuming practice of making similar designs by hand, as it allowed for greater delicacy, precision, and closeness of the line, as well as allowing greater speed.
A style of engraved decoration that, in jewelry and objects of vertu, is made on metal by means of an engine-turning lathe having an eccentric motion that can cut a variety of patterns, the most elaborate of which are done with a guide called a 'rosette', hence 'rose-engine-turned'. (It is different from the meaning of 'guilloche' as decoration on ceramic ware, and also on some Costa Rican jade jewelry, which is a border pattern in the form of 2 or 3 bands, twisted alternately over and under each other in continuous series, in such a manner as to leave openings that are sometimes filled in with ornamental motif; it was a speciality of Sèvres in the second half of the 18th century).
The French term for the work is guillochage. When such engraving is covered with transparent enamel that reveals the pattern beneath, as found on some pieces by Carl Fabergé, it is called tour à guillocher.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson