Among the casualties of the Great Recession is the faith many Americans once had in a concept called the American dream.

It hasn’t made much of a comeback even though the economic expansion coming out of the Great Recession is about to turn the ripe old age of six. By some measures, such as the new jobless claims data from last week, the job market hasn’t been this solid for 15 years.

Yet in poll after poll, Americans say achieving the dream seems out of reach. And it’s not just because wages haven’t grown much and housing has gotten more expensive.

The idea of the American dream is so powerful, and apparently so broadly understood, that it seems almost silly to try to define it. Pollsters usually don’t, which is how we get a result like the widely reported CNN/ORC International poll of last summer, which found that 59 percent of Americans agreed with the statement that “the American dream has become impossible for most people to achieve,” like we were all supposed to know exactly what that meant.

There’s also an actual number published monthly called American Dream Composite Index, which fell in April to 64.52, about what it was in September 2011.

It’s meant to be a real indicator, produced by serious people at Xavier University in Cincinnati. The part of its website that explains what it is said only that the composite “measures the extent to which people living in the United States achieve the American dream.”

A search for help on coming up with a more complete definition of the American dream led to John Archer, a University of Minnesota professor who’s long thought about it.

Archer is a professor of cultural studies, not an economist, and he comes to the topic from a background in architectural history. That led him to studying and then explaining the commitment of many Americans to owning their own single-family house in the suburbs.

That, to him, is the easiest way to characterize what the American dream has come to mean over the course of several generations. The reason to work hard and “get ahead” financially is to end up with enough money to buy — and keep — a single-family house.

That explains why a piece of federal housing finance legislation before the collapse of the housing market was simply called the American Dream Downpayment Act of 2003.

And a few years later, after the bubble burst, a big nonprofit went from city to city putting together events for homeowners, mortgage servicers and credit counselors to sit together to restructure mortgages in default. It called its campaign the “Save the Dream Tour.”

That economic downturn of 2008 and 2009 certainly battered the hope that underlies the American dream. Many Twin Cities homeowners saw their net worth evaporate as the median Twin Cities sales price for a house fell more than 41 percent from the peak value to the trough, according to the real estate information firm Zillow.

As Archer put it, “if somebody looked at the dream from that vantage point, it looked absolutely hopeless.”

What’s happened more recently, he said, is that “a lot of industriousness” has gone into trying to make this great-sounding concept continue to mean something important, perhaps by making it be about something other than buying a house.

Archer said it would be helpful if the people who want to debate what the American dream means would acknowledge the structural changes in the economy since the 1950s, what he thinks may be been the heyday of the traditional American dream.

Median household incomes were far lower than they are now, and the average size of a new house in 1950 was just 983 square feet, but a sense of optimism about achieving that style of living was broadly shared.

One good way to go about it was to get hired by a big company and stay put, Archer said. A white-collar job might mean topping out in middle management, and for blue-collar workers covered by a union contract the outcome was largely the same: A modest but stable lifestyle, with a pension to keep the bills paid after their working lives.

Since then the average size of a new house has ballooned to nearly 2,700 square feet, so Americans clearly can afford bigger houses than in the 1950s. But it’s come at the cost of stability.

Job losses in recessions up through the 1980s happened mostly to blue-collar workers, but beginning with the recession of the early 1990s job losses hit managers and other white-collar workers hard, too. In 1973 well over half of men 40 to 64 had been with the same employer at least 10 years, but that kind of job tenure has since fallen dramatically.

Retirement security also evaporated since then, with pension plans abandoned in favor of defined contribution plans like a 401(k).

Along with a few other factors, the reliance on a do-it-yourself retirement through a 401(k) means more than half of today’s working households can expect to have a lower standard of living after retirement than they did when they were working, according to a late 2014 study from the Center for Retirement Research at Boston College.

It’s that far shakier economic ground under the feet of American workers in 2015 that’s led to TV commercials like those for American Family Insurance. “Your dream is out there,” the company says. “Go get it. We’ll protect it.”

These dream ads have an upbeat tone and likable stars such as pro football player Russell Wilson, but Archer is clearly right that they are not trying to tap into the thrill of attaining your dream. They are really tapping into the fear of losing what you already have.

That’s a vision of the American dream — with luck and hard work, you won’t go backward — that’s much worse than Americans merely losing faith in the dreams of their parents and grandparents.