Could anything be more fundamentally American than the right to life, liberty, and the pursuit of happiness? Probably not. But the definition of that last, elusive word — happiness — seems to be changing dramatically as the Millennial generation enters adulthood.

Ask your parents or grandparents to define happiness and they’ll surely talk about love, friends, and family. Next, they’ll probably mention succeeding in their chosen career, owning a nice home, and having a solid nest egg.

But ask a Gen Y, and the definition of success and happiness may sound quite different. As journalist Hannah Seligson recently wrote of her peers in the Washingtonian, “Instead of a steady job, they want a meaningful one that serves a larger purpose or fulfills a personal passion. And instead of settling down with a spouse and mortgage, they want more years of freedom to chase career dreams and explore different paths before they have to make tradeoffs.”

For Millennials, things like climbing the corporate ladder, socking away money for a home, and building up retirement savings have one serious drawback: they take a lot of time. And, in my experience, Millennials don’t like to wait.

Perhaps because so many of their parents showered them in self-esteem, or perhaps because they witnessed the horrors of 9/11 at a young age and learned that life can be too short, Millennials tend to have a carpe diem philosophy. They want it all, and they want it now.

While detractors call this “entitlement,” I sometimes admire Millennials’ belief that paying one’s dues is a waste of time. Millennials want to explore multiple careers, relationships, lifestyles, and technologies without committing to any one path too soon. For a generation with a life expectancy of 100 or more, why not live big in your 20s?

And, while the recession has been particularly harsh on this generation, even a bad economy hasn’t seemed to curb Millennials’ enthusiasm for personal satisfaction. Even with dangerous student loan and credit card debt, most college graduates I meet still place primary importance on finding a career and lifestyle they love — even if it means moving back home with Mom and Dad for a while.

Not all young people are fortunate enough to have a parental safety net, but those who do are taking full advantage, “waiting out” the recession with unpaid internships, part-time work, or additional schooling until they can find careers and lifestyles they love. And with free and low-cost resources like Facebook, Spotify, Hulu, GChat and a million other apps, all you need is a smartphone or laptop to have a lot of fun and a thriving social life.

Maybe it’s a response to the economy: If we can’t find jobs anyway, we might as well enjoy ourselves. But I think there’s more to the Millennials’ new definition of success and happiness. I think they want things members previous generations wanted, too — freedom, choices, meaning, passion. They just didn’t prioritize them over money and security. Millennials do.

So, how will this all shake out in 2012 and beyond? Will Gen Y “settle down” as they age and begin to define success as previous generations have? Will they prove that if you do what you love the money will follow? Or will this generation forever change the meaning of “well off” and put a higher value on personal satisfaction than on material wealth? Whatever the outcome it will happen soon, because that’s the only timeframe Millennials know.