Live your own life without anybody telling you how to live it and meaning by making your own decisions too on the way. And to just live life to the fullest of the best to your knowledge in being happy it's the most important thing. Just to simply be happy in life.
I'm not sure if it's etched in stone. I would like to think it's individually defined. The average, being "middle class," American used to be: white, 2 parents, 1 income, 2.5 kids, a golden retriever (or family dog), a white picket fence, a neighborhood who all cared and shared BBQs and family outings who watched out for each other, baseball games, great schools within 3 miles of the home, women who wore appropriate dresses everyday and did their makeup nicely, men who wore a jacket and tie to work, one car, and a feeling of commodity and caring among all. People who cared about people and respected where they lived, their families, friends, and careers/jobs. Now, I'm not really sure what people make of the "American Dream." I'm sure it's changed since the Leave it to Beaver days.
The American Dream that I was sold as child had to do with having the opportunity to accomplish something, create something or invent something or discover something of real value to society as a whole. To make a contribution that society would recognise and reward. The American Dream being sold today seems to be about getting rich quick and it doesn't matter how. Forget the accomplishment, just get to the rewards.