Basse taille (French), literally, shallow cut.
The technique of decoration by enamelling on a metal base in which the design was first made in several levels by chasing, carving, engraving or stamping (or, in later examples, by engine-turning) and then the surface was covered with transparent or translucent coloured enamel (but without any partitions to separate the colours) that was then fused by frying.
The varying depths of the depressions of the design resulted in different tones of the enamels and thus enhanced the effect of apparent intaglio relief. The enamel decoration, after firing and polishing, was smooth and level with the metal surface. The metal base was usually gold or silver. The enamel was sometimes of different colours, but was most effective when of a single colour (usually blue or greem).
The technique is said to have originated in Italy in the late 13th/14th century, and thereafter was used elsewhere on the Continent, especially in the Rhineland and France, and in England. Sometimes called 'translucent enamelling'
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson