Cask strength is a term generally used for whisky that has an alcohol level above 50% ABV. Many people believe that this is whisky in it's true form, as it is bottled directly from the cask with nothing added. Most whiskies have spring water added before bottling to bring the alcohol level down to between 40-45%. This is known as 'cutting'. This makes the whisky more approachable to the majority of drinkers, who find higher alcohol levels too intense or strong. However, others drink cask strength whisky for these very reasons and the intensity of the flavours.
Here's a little known whisky secret ... An easy way to find out if a whisky is cask strength or not, is by shaking it in the bottle. If the bubbles disappear almost instantly, then the whisky has an alcohol level below 50% and is not cask strength. However, if the bubbles take longer to disappear, then the whisky is over 50% and cask strength.