Vermeil is a combination of sterling silver and gold commonly used as a component in jewelry. A typical example is sterling silver plated with 18k or 24k gold. To be considered vermeil, the gold must be at least 10k (42%) and be at least 2.5 micrometers thick. (A micrometer is one thousandth of a centimeter.)
The plating on vermeil does wear off fairly quickly compared to gold fill; however, having sterling silver underneath makes it safe for people with extremely sensitive skin or extremely severe metal allergies. It is also appealing to those who prefer items comprised solely of precious metals, since the base metal used in gold fill can vary.