First brewed in England and exported for the British troops in India during the late 1700s. To withstand the voyage, IPA’s were basically tweaked Pale Ales that were, in comparison, much more malty, boasted a higher alcohol content and were well-hopped, as hops are a natural preservative. Historians believe that an IPA was then watered down for the troops, while officers and the elite would savor the beer at full strength. The English IPA has a lower alcohol due to taxation over the decades. The leaner the brew the less amount of malt there is and less need for a strong hop presence which would easily put the brew out of balance. Some brewers have tried to recreate the original IPA with strengths close to 8-9% abv.
The American IPA is a different soul from the reincarnated IPA style. More flavorful than the withering English IPA, color can range from very pale golden to reddish amber. Hops are typically American with a big herbal and/or citric character, bitterness is high as well. Moderate to medium bodied with a balancing malt backbone.
Take an India Pale Ale and feed it steroids, ergo the term Double IPA. Although open to the same interpretation as its sister styles, you should expect something robust, malty, alcoholic and with a hop profile that might rip your tongue out. The Imperial usage comes from Russian Imperial stout, a style of strong stout originally brewed in England for the Russian Imperial Court of the late 1700s; though Double IPA is often the preferred name. (BeerAdvocate)