Plique-á-jour is a French term for enamel that has no backing. A design is outlined in metal and filled with various colored transparent enamels giving the effect of a stained glass window. This technique was used in the 15th century by Benvenuto Cellini and was rediscovered in France in 1900. This style of enameling was very popular during the Art Nouveau period.
Plique-á-Jour from the Antique Jewelry University
http://www.langantiques.com/university/index.php/Plique-%C3%A1-Jour