New Delhi, July 30 -- As a child, Heidi Barley watched her family pay for groceries with food stamps. As a college student, she dropped out because she couldn't afford tuition. In her twenties, already scraping by, she was forced to take a pay cut that shrunk her salary to just $34,000 a year.

But this summer, the 41-year-old hit a milestone that long felt out of reach: She became a millionaire.

A surging number of everyday Americans now boast a seven-figure net worth once the domain of celebrities and CEOs. But as the ranks of millionaires grow fatter, the significance of the status is shifting alongside perceptions of what it takes to be truly rich.

"Millionaire used to sound like Rich Uncle Pennybags in a top hat," says Michael Ashl...