People frequently ask, "What is single malt whiskey?" in liquor stores, restaurants and bars. The answer to "What is single malt whiskey?" is deceptively simple. Single malt whiskey is whiskey that has been made from barley (almost always), water and yeast.
In single malt whiskey only one grain is used, therefore word single can be used to describe the malt whiskey. 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 whiskey.
Single malt whiskey is most often associated with the whiskeys of Scotland, but Japan, Canada, Germany, Taiwan, India, France, the United States and many other places do produce single malt whiskeys.
Additionally, single malt whiskey is the product of one distillery. If you took a single malt whiskey from Distillery A and mixed it with Distillery B, you no longer have a single malt whiskey. Essentially, it would be a vatted whiskey.