A method first begun by the Glenmorangie Distillery wherein the whisky is aged for a certain amount of years in one type of barrel, usually ex-bourbon barrels, and then rested in another type of barrel (most prevalently Port, but also Sherry, Madeira, Burgundy, Sauterne, and others) for varying amounts of time. This method imparts flavors and colors from the original barrel's contents to the whisky resulting in a unique expression.
See bushmills for irish whiskies that use this method too.