Extra Dry is a term generally used to describe a measure of sweetness in a sparkling wine; the term is used in many different types of bubblies from Champagne to inexpensive sparkling cuvées.
Extra Dry sparklers are actually lightly sweet, some with a trace of sweetness, some medium-sweet. The term extra dry derived from the unusual use of particular terms to describe Champagne, as a Dry Champagne is actually quite sweet! Thus the term Extra Dry was created to describe a Champagne that was drier than a Dry Champagne ; the driest style- even drier than Extra Dry - is Brut.
While an extra dry sparkling wine has a touch of sweetness, it is dry enough for many individuals, so it can be served at the dinner table with any number of foods, from salad to seafood.
While the term extra dry is almost always associated with sparkling wines, it is also used to describe other beverages, such as sake.