By taking a normal American Stout and doubling down on the roasted flavors, alcohol, and hops American brewers have created the Double or Imperial Stout.
Borrowing both from the American Stout and the Russian Imperial Stout, the Double Stout is almost always either a very dark brown to pitch black. A large amount of very dark, roasted malts, aromatic hops and alcohol are characteristic of the style. Although the flavors can be very intense, the beer is usually balanced, and sometimes aged in whiskey or oak barrels.
The term "Imperial" comes from Russian Imperial Stout, a style of strong beer originally brewed in England for the Russian Imperial Court of the late 1700s. Best served at cellar temperature, about 55-57 degrees, in pint glass, a beer snifter, or a large wine glass.