Often, the dividing lines are tied to major global events, such as the end of World War II in the case of the Baby Boomers. And, sometimes, a generation’s identity, rightly or wrongly, is pinned to a more amorphous sociological trend, such as the existential angst that characterized the grunge rockers of Gen X.

Gen Y came after Gen X, hence the Y, which doesn’t tell us much of anything. So, maybe Millennials is a more descriptive term.

What do we know about these “digital natives” for whom a smart phone is just a phone? We know it’s a big group, but we don’t know how big, because the lines between generations are subjective.

“There’s an arbitrariness to the designation of generations, and they are not of uniform size,” says Leanne Lachman, president of Lachman Associates, an independent real estate consulting company. “The birth years for Gen Y are 1978 to 1995 (a 17-year span), while Gen X is 1965 to 1977 (a 12-year span). And so, Gen X is much smaller: Think of it as a sandwich, where Gen X is the very thin filling between the bread of Gen Y and Baby Boom.”

The phrase Generation Y was believed to have been coined by marketing news publication Ad Age in 1993. It used 1982 as a starting date, as do influential demographers William Strauss and Neil Howe, authors of Generations: The History of America’s Future.

Lachman’s starting date of 1978 is one year ahead of MetLife Market Intelligence, which puts Gen Y as those born between 1977 and 1994. All of these variations make it hard to pin down an exact number—are there 78 million, 80 million, 85 million?

And when you’re talking about numbers this high, does precision really matter? The bottom line is, this generation is big and, given immigration trends, getting bigger every day.