Single barrel whiskey (or single cask whiskey) is a premium class of whiskey in which each bottle comes from an individual aging barrel, instead of coming from blending together the contents of various barrels to provide uniformity of color and taste. Even whiskeys that are not blends may be combined from more than one batch, or even from differing years to achieve consistency. The whiskey from each barrel is bottled separately, with each bottle bearing the barrel number and in most cases the dates for the beginning and end of aging. Each barrel is believed to contribute unique characteristics to the finished whiskey.There has been some recent controversy over whether single cask whiskeys are indeed all from single casks. Whiskeys sold by Scottish distilleries such as Ben Nevis and GlenDronach as "single casks" have been revealed to be vattings of multiple barrels, which may have been of different kinds, with the "single cask" designation referring only to the final cask for maturation. In the absence of specific regulation regarding this language, it is not clear to what extent this practice is prevalent in the industry as a whole.
Single barrels may further be at cask strength or non-chill-filtered to further avoid adulterating the taste of a cask. Wikipedia