The Brewer's Association defines an Imperial IPA as an IPA with color that is straw to medium amber, 6.0%-8.4% alcohol, with hop aromas and flavors that are very high, but not aggressively bitter.
Pale ale is a type of beer that is brewed with mostly pale malts for a more equal malt-to-hop ratio. “IPA” stands for India Pale Ale, a hoppy style of beer within the pale ale category. Double IPAs, also called Imperial IPAs, are a much hoppier style of IPAs with alcohol content above 7.5 percent by volume.
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! If you like the basic style (stout, IPA, etc.), chances are that you'll enjoy the amped up imperialized version as well.