出處:Natural cask strength Blended whiskies are almost always "reduced" (cut with water) to 40% or 43% Alcohol by Volume (80 or 86 Proof). Single malt whiskies are often "reduced" to similar alcohol levels, although some distillers prefer 46% Alc. (92 Proof). Natural cask strength is the strength at which the alcohol naturally exists in the cask. Distilleries like to put spirit in the barrel at 63.5% Alcohol (127 Proof) as it's believed to enjoy the best barrel interaction at that strength. During the maturation process alcohol will naturally evaporate through the pores in the oak cask and the alcohol level will naturally decrease. Sometimes the alcohol level decreases quickly while sometimes it decreases slowly. It's all dependent on the single cask and, it's believed, its location in the warehouse. We only bottle at natural cask strength for Single Cask Nation (you're welcome to "reduce" the Alc. in the privacy of your own home).
'natural cask' is an umbrella for anything that's not sherry (mainly first fill) or other wine treated/finished. As specified on the MMA page, 'whisky matured exclusively in 'untreated', regular cask(s)'. In short, whiskies that bear less influence from the cask(s)' previous content than, for example, whisky from first fill butts. It's no 'official' denomination
"Natural Cask" = 波本舊桶 or 重灌桶 (ex-bourbon or refilled casks)