The origins of India pale ale are similar to those of Bombay gin, which was mixed with quinine to combat malaria, in that there’s a real, tropical reason for the name:

After the British East India Company had established itself in India… it had a large number of troops and civilians demanding beer… Ships typically left London, cruised south past the equator along the coast of Africa, rounded the Cape of Good Hope and then crossed the Indian Ocean to reach Bombay, Calcutta, and Madras. The temperature fluctuations were huge, it was a very long trip (about 6 months) and the rough waters of southern Africa resulted in an extremely violent voyage…

Early shipments to India contained bottled porters, the favorite beer in London, which generally arrived flat, musty, and sour… Hodgson took his pale ale recipe, increased the hop content considerably, and raised the alcohol content. The result was a very bitter, alcoholic, and sparkling pale ale that could survive the challenges of travel and shelf life in India. [Link]

High hop levels can preserve a beer’s flavor in two ways: they have a limited ability to protect beer from spoilage by some microorganisms, and, more importantly, their bitterness can mask stale flavors. While the beer arriving in India would certainly have suffered from oxidative staling during the long voyage, it could still taste acceptable because of the masking effect of alcohol and hops. [Link]

There’s nothing the Brits did that the Americans can’t do bigger and better:

… the IPA style has become very popular with American microbreweries, where it has evolved into a distinct variant sometimes called American Indian Pale Ale… American brewers have also altered the style to increase the bitterness of the beer to well over 100 IBU’s and the alcohol levels up to 10 or even more than 20 percent ABV. These beers are referred to as American Double India Pale Ales or Imperial India Pale Ales. [Link]

The World Beer Championships define it more precisely:

English-Style India pale ales are characterized by intense hop bitterness and high alcohol content… The use of water with high mineral content results in a crisp, dry beer. This pale gold to deep copper-colored ale has a full, flowery hop aroma and may have a strong hop flavor (in addition to the hop bitterness). India pale ales possess medium maltiness and body. Fruity-ester flavors and aromas are moderate to very strong…

Imperial or Double India Pale Ales have intense hop bitterness, flavor and aroma. Alcohol content is very high and notably evident. They range from golden to amber in color. The style may use any variety of hops. Though the hop character is intense it’s balanced with complex alcohol flavors, moderate to high fruity esters and medium to high malt character. While hop character may be very intense, it may not be harsh in quality. Imperial or Double India Pale Ales have medium-high to full body. [Link]

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p>Note that (American) Indian Pale Ale is not (American Indian) Pale Ale.

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p>Finally, Imperial Pale Ale used to refer to exports to Russia:

… brewers that wanted to export pale ale from England to Russia were facing the same problem of avoiding the rotting of the beer, thus they increased the hop and the alcoholic strength of the beer, obtaining the so called Imperial Pale ale. Ultimately the exports to Russia had to be stopped for political reasons. Brewers exporting to Russia eventually switched to India, and so “Imperial Pale ale” and “India Pale ale” became synonymous. [Link]

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