The quest for more of the India pale ale flavor has led them to the imperial India pale ale, a stronger version of the American IPA, which boasts even more hoppy flavor, aroma and bitterness. Imperial India pale ale is darker in color than the American IPA, substantially more bitter, and high in alcohol by volume.
What's a Double IPA? Double IPA's, also called Imperial IPA's, take a regular IPA and amplify it with a bigger malt and hop profile. The alcohol content usually greater, too. Breweries have taken it to another level developing Triple and Quad IPA's that are much less commercialized and a higher alcohol content.
The term “imperial” was used in the 1800's for beer (usually stout) that was brewed in England but then shipped to Russia – specifically the imperial court of Russia! Brewers later started using the term more generally to indicate their top-of-the-line luxury beers.