People frequently ask, "What is single malt whisky?" in liquor stores, restaurants and bars. The answer to "What is single malt whisky?" is deceptively simple. Single malt whisky is whisky that has been made from barley (almost always), water and yeast.
In single malt whisky only one grain is used, therefore word single can be used to describe the malt whisky. The barley (or other grain) is malted, which is a process of starting the barley seed to sprout by soaking it in water, and then interrupting the process via heat. Unmalted barley (think of it as whole grain or seed barley) can and is used in whiskies, but those whiskies obviously wouldn't be single malt whisky.
Single malt whisky is most often associated with the whiskies of Scotland, but Japan, Canada, Germany, Taiwan, India, France, the United States and many other places do produce single malt whiskies.
Additionally, single malt whisky is the product of one distillery. If you took a single malt whisky from Distillery A and mixed it with Distillery B, you no longer have a single malt whisky. Essentially, it would be a vatted whisky.
Famous single malt whiskies include the Glenlivet, Glenfiddich, Balvenie from Scotland, McCarthy's from Oregon, Amrut from India, Kavalan from Taiwan, Blaue Maus from Germany and Penderyn from Wales.