A few days ago, I read Better Half a bit of something I'd been writing about cooking terms, and my mention of a roiling boil completely flummoxed him. Not sure whether I'd stumbled on a dialectal difference or whether BH just had a poor vocabulary when it came to cooking, I asked my boss, a historical linguist who also professed ignorance of the word roil. So, dialectal difference then--BH's vocabulary is vindicated!
To roil is to move water so that whatever is in it (sediment, etc.) gets stirred up. By metaphorical extension, it can mean to perturb or upset. A roiling boil, then, is the type where your eggs knock against your pan.
OED's first citation of roil is from 1590, but most citations past 1700 are American. The origin is obscure, possibly from a rare French verb. Development of the adjective roiling seems particularly American, with the first citation in 1967.
It's a common enough word in American recipes, with an estimated 9300+ Google hits for roiling boil:
To purify questionable water, bring it to a roiling boil and keep it there for 10 minutes at least.
Bring to a roiling boil, reduce to simmer and cook until the meat is tender...
The sea and emotions frequently roil as well.
The British equivalent for roiling boil is fast boil [updated link].
The lack of roiling boil in BrE reminds me of another American -oil word, broil.
Playing Scrabble recently, my opponent The Postman was unsure of broiled, but played it figuring that it must be related to embroiled. This reminded me of one of the first times I watched "Who wants to be a millionaire?" in the UK. The million pound question was (approximately, from memory):
The American word broil means:
(a) bake (b) boil (c) grill (d) braise
I could've won a million pounds, except that I probably would've wiped out on a cricket question long before getting to the mil.