Any spirit which contains only one malt. Scotch is often sold as a single malt which allows the taster to enjoy the various regional and distillery characteristics of various malts. These spirits may be intermixed among oak barrels and ages to preserve consistency, but will all have originated from a consistent recipe and process.
Another type of single malt is sold as cask strength and is normally a malt from a single cask, unmixed and undiluted before bottling.
Single malt is a term used to describe a whisky distilled from a barley malt wash. Single malt refers to the fact that the whisky is not blended with any other malts before being bottled, thus retaining unique regional characteristics.