Malt whisky is whisky that is made from a fermented mash produced primarily from a malted grain. Unless otherwise indicated, it is generally assumed that the primary grain is barley, although whisky is also made using malted rye or other grains. A whisky made from malted rye is usually called a rye malt whisky instead of just a malt whisky.
If the product is made exclusively at a single distillery (along with other restrictions), it is called a single malt whisky.
Laws
The exact definitions of "malt whisky" and "single malt whisky" and the restrictions governing their production vary according to the regulations governing the marketing of whisky in the local jurisdiction.
In the United States Code of Federal Regulations, the Standards of Identity for Distilled Spirits define a "malt whisky" as a whisky produced at an alcohol by volume (abv) level not exceeding 80% from a fermented mash of not less than 51% malted barley grain and stored for aging in charred new oak containers at less than 62.5% abv. The regulations also define a "rye malt whisky" in a corresponding manner using rye grain. If such a whisky has been aged for at least two years, contains no added coloring and flavoring, and has not been blended with neutral spirits or other types of whisky, the additional adjective straight can be applied, as in "straight malt whisky" and "straight rye malt whisky". If the whisky is not totally straight but at least 51% of its content is "straight malt whisky", the word "malt" can still be used, but the product must be described as a "blend", as in "blended malt whisky" or "malt whiskey – a blend". There is no aging requirement for the parts of a blended whiskey other than the part that is straight whiskey – e.g., the product may contain neutral spirits that have not been aged and have been made from other types of grain.
Canadian whisky regulations are generally more liberal, allowing the addition of flavoring agents as well as caramel, and not specifying the use of any particular type of grain when making malt whisky. There is no maximum limit on the alcohol level of the distillation or aging for Canadian whisky, so the bulk of the distilled content (often more than 90 percent) may be neutral spirits or near-neutral spirits rather than "straight" whisky. Similar to the regulations for the aging of Irish and Scotch whisky, the aging requirement for Canadian whisky is three years, and the use of new barrels is not required.
Under Irish whiskey regulations, malt whiskey is to be distilled in a pot still from a mash of 100% malted barley, which may be peated or unpeated in character (though typically unpeated malt is used). As with all Irish whiskeys, it must be matured for at least three years in wooden casks (which may have been previously used), and must contain a minimum of 40% alcohol by volume, with the only additive permissible being caramel colouring (E150a).
In Scotch whisky regulations, malt whisky must be made using at least some barley and must be aged for at least three years, the use of new barrels is not required, the distillation and aging can use higher levels of alcohol by volume (which can result in a "lighter" but less flavorful whisky), and (E150A) caramel coloring can be added (but no other additives are allowed).