An ale of significant alcoholic strength, bigger than strong bitters and brown porters, though usually not as strong or rich as barleywine. Usually tilted toward a sweeter, maltier balance. "It should be a warming beer of the type that is best drunk in half pints by a warm fire on a cold winter's night" Michael Jackson.
Strength and character varies widely. Fits in the style space between normal gravity beers (strong bitters, brown porters) and barleywines. Can include winter warmers, strong dark milds, strong (and perhaps darker) bitters, blended strong beers (stock ale blended with a mild or bitter), and lower gravity versions of English barleywines. Many English examples, particularly winter warmers, are lower than 6% ABV.
Gale's Prize Old Ale, Burton Bridge Olde Expensive, Marston Owd Roger, Greene King Olde Suffolk Ale , J.W. Lees Moonraker, Harviestoun Old Engine Oil, Fuller's Vintage Ale, Harvey's Elizabethan Ale, Theakston Old Peculier (peculiar at OG 1.057), Young's Winter Warmer, Sarah Hughes Dark Ruby Mild, Samuel Smith's Winter Welcome, Fuller's 1845, Fuller's Old Winter Ale, Great Divide Hibernation Ale, Founders Curmudgeon, Cooperstown Pride of Milford Special Ale, Coniston Old Man Ale, Avery Old Jubilation
You can view the complete style details on the BJCP website. The 2008 style guide is obsolete but there is a similar 2015 style that you can view below...