Vermeil (pronounced ver-may) is 24k gold on top of a sterling silver base. Its a gold plating technique, but where gold plated jewelry can use various base metals and has no minimum depth, vermeil jewelry always uses hypo-allergenic sterling silver as a base metal, and is at least 2.5 microns thick.
Sometimes called silver-gilt, the techniques that eventually evolved into vermeil jewelry-smithing date to at-least the 4th century BCE. Ancient romans overlayed hammered gold leaf, Incan's used depletion gilding, chinese "ormolu" used gilt-bronze and korean's "keum-boo" was a method for silver-gilding. In the 18th century French Artisans evolved these metal working techniques to eliminate the use of mercury and gold vermeil was born.