refers to whisky bottled at the alcoholic strength it had in the cask, without any dilution with water. The alcoholic content of cask strength varies but is usually around 50% to 65% abv.
Normally whisky is reduced to 40% or 43% volume alcohol before it's bottled. A whisky of cask strength is bottled at the alcoholic strength as it is taken from the cask. According to its age and the evaporation that took place during maturation it may have 65% to 45% Vol. alcohol.
Is said of a whisky which has not been diluted with water and has been bottled at its original abv when emptied from the cask.