I've seen the adjective "Imperial" tacked on at the beginning of a number of beer styles recently: Imperial Stout, Imperial Brown Ale, Imperial Red Ale, etc.
What does it mean exactly, is it just higher alcohol content or something like that?
English brewers, brewing beer for the Russian Czar's court brewed a beer with a high ABV to try and impress the royalty of Russia. Lots of hops were added to balance the malt, and survive the journey to Russia. This was the original Russian Imperial Stout.
As will all things, American Craft breweries have now taken this name to put a spin on any beer that is higher alcohol than its traditional counterpart. It may also contain a lot more hops than normal.
Examples, Imperial IPA, Imperial Pilsner, Imperial Brown ale... you get the idea.
It is part marketing and part tradition.
The term has origins in high gravity and hopped porters exported from England. The BJCP 2008 Style Guidelines has this to say about the Russian Imperial Stout:
Brewed to high gravity and hopping level in England for export to the Baltic States and Russia. Said to be popular with the Russian Imperial Court. Today is even more popular with American craft brewers, who have extended the style with unique American characteristics.
"Imperial" is this brewing world's "EXTREME!" Higher alcohol, more hops, more malt, more everything! Quoth the BJCP about Imperial IPA:
A recent American innovation reflecting the trend of American craft brewers “pushing the envelope” to satisfy the need of hop aficionados for increasingly intense products. The adjective “Imperial” is arbitrary and simply implies a stronger version of an IPA; “double,” “extra,” “extreme,” or any other variety of adjectives would be equally valid.
I always consider Imperial to mean "bigger" (higher abv). A more extreme version of the style with more of various ingredients. There are larger additions of specific ingredients generally based on the style, such as an Imperial IPA probably is more heavily hopped. It also seems that the word "double" could easily be interchanged most of the time.
im·pe·ri·al (m-pîr-l) adj. 1. Of, relating to, or suggestive of an empire or a sovereign, especially an emperor or empress: imperial rule; the imperial palace. 2. Ruling over extensive territories or over colonies or dependencies: imperial nations. 3. a. Having supreme authority; sovereign. b. Regal; majestic. 4. Outstanding in size or quality.
Outstanding in size or quality seems to apply to beer quite nicely.
Surface meaning when picking up a bottle at the store: more. More hops, more malt - more ABV.
Additionally, it also refers to some of the beer's history. Imperial Indian Pale Ale relates to the exporting of beer from England to India during their imperial reign (fact check?).
Similarly, Imperial Stouts were exported from England to Catherine II of Russia.
I believe a beer must be at least 8.5% alc or 9% to be an Imperial beer. For example an IPA with 8% alc is not imperial, and Imperial IPA would be I believe at least 8.5-9% and up.