A person who has collected at least one million miles with a single airline's frequent flier program.
When you have accumulated 1 million miles you qualify for life membership at the first tier 'Silver Medallion' level with Delta and 'Gold' with American. ... Watch out for expiration dates. It's getting hard these days to retire as a mileage millionaire.
Roger Collis, "From Web to World," International Herald Tribune, March 9, 2001
First there was the landed gentry. Now comes the airborne equivalent — 'millionaires' whose currency is frequent-flier miles.
In the 12 years since American Airlines introduced the frequent-flier program and other carriers quickly matched it, these people have stockpiled a million or more miles, redeemable for free trips or upgrades to first or business class.
Stories about how people have acquired-and spent-their miles have reached mythic proportions. ... But there are plenty of mileage millionaires with verifiable stories. Indeed, says Randy Petersen, editor of Inside Flyer magazine, more than 100,000 people have accumulated a million miles at one time or another.
—Deborah L. Jacobs, "They're the Donald Trumps of the Skies," Chicago Tribune, December 19, 1993
American Airlines introduced the first frequent flier program back in 1981, but it took 12 years before the term mileage millionaire was seen in print, as the earliest citation shows. In case you're wondering, the most recent data I could find (from November, 2000) pegs the current number of U.S. mileage millionaires at 121,000.