Any wine that is made with (usually, at least in the U.S.) 75% of any particular grape variety (don't be dorky and say "varietal" when you mean "variety," as in a type of grape). In most of the new world, including the U.S., wine is named for the grape variety that's in the bottle, i.e., cabernet sauvignon, chardonnay, merlot. In Europe, wine is usually named for the place it comes from, i.e., Burgundy, Bordeaux, Chablis, etc. Many people find this confusing. Not Winegeeks.
A wine made of and named for predominantly one grape variety, such as Cabernet Sauvignon. Varietal labeling is far more common for new world wines than those of old world, which are traditionally named for a place like Bordeaux. Varietal regulations vary from country to country. In America, the minimum required to name the wine for a variety is 75%.