India Pale Ale (IPA, also known as Imperial Pale Ale), is a distinct style of beer and is characterized as a sparkling pale ale with a high level of alcohol and hops, thus having an increased bitterness.
IPA descends from the earliest pale ales of the 17th century, when the term "pale ale" probably simply distinguished ales which were light in color compared with brown ales of that day. By the mid-18th century, pale ale was mostly manufactured with coke-fired malt, which produced less smoking and roasting of barley in the malting process, and hence produced a paler beer. One such variety of beer was October beer, a pale well-hopped brew popular among the landed classes, who brewed it domestically; once brewed it was intended to cellar two years.
Double India Pale Ales (also abbreviated as Double IPAs or IIPAs) are a strong, very hoppy style of pale beer. Also known as Imperial IPAs, perhaps in reference to the Russian Imperial Stout, a much stronger version of the English Stout, these beers are essentially India Pale Ales with higher amounts of malt and hops. Double IPAs typically have alcohol content above 7% by volume. IBUs are in the very high range (60+). Such "style" labels can seem arbitrary however, since the aforementioned Ballantine IPA in its original formulation was certainly well above these benchmarks. (Full article...)