A Tastevin is a relatively small, shallow silver cup or saucer, with a thumb-handle - which was traditionally (and for some still) used by winemakers and négociants when judging the maturity, quality, clarity and the taste of a wine - (typically from barrel in the cellar).
The saucer-like cups were originally designed by a winemaker in Burgundy - where a high-level wine society called the 'Confrerie des Chevaliers du Tastevin' - was named after the tasting cup. Enabling them to judge the clarity and colour of wine that was stored in dim, candle-lit wine cellars. The Tastevin cup can vary in size but is typically 7 - 8 cm in diameter. The wine would be poured to a shallow level - and over the brightly reflecting silver, tasted and swirled by the connoisseur and spat out into a bowl or even on the cellar floor.

 

By definition a taster, 'tasse a vin' or tastevin would only hold a small amount of the wine. It needed to be made of a material strong enough to withstand the rigors of daily use as well as being made of a material (Sterling silver) that would not taint the wine in any way.
Regular wine glasses are too deep to allow for accurate judging of the wine's colour in such dimly lit cellars. Since it is relatively flat like a saucer, it is almost useless for smelling the wine. Tastevin are designed with a shiny dimpled inner surface. Often, the bottom of the cup is convex in shape. The dimples, convex bottom and the shiny inner surface catch as much available light as possible, reflecting it throughout the wine in the cup at various angles at once, making it possible to see through the wine. Clarity is less of an issue than it used to be in wine, and with the advent of modern electric lighting, glasses are much more effective - so the tastevin has mostly been relegated to a novelty.
Although some 'Sommeliers' can be seen to wear them around on a ribbon or chain around their neck as a sign of respect to tradition.
There are references to the use of Tastevin's in old manuscripts of the 14th and 15th centuries and the earliest English tastevin is dated 1603. The tastevin remains today as the acknowledged 'Ceremonial Symbol' of Burgundy - France.