There's a general consensus among marketers when it comes to defining 'affluent': Families or individuals with over $1M in investable assets. It's a numerically small group that is heavily targeted, especially by luxury brands.
The tier below them are known as the Mass Affluents. Depending on who you listen to, this group is defined variously as those who save more than they spend (Wikipedia), people withbetween $50-$250,000 in investable assets (not including their homes, Merrill-Lynch). Nielsen uses a higher standard, defining this group as having between $250k-$1M in investable assets.
According to Nielsen, there are about 13 million Mass Affluent households in the U.S., representing about 11% of all households. (This group, incidentally, holds over 1/3 of the country's wealth and had an average income of over $102k in 2011, i.e., their household income was more than 50% higher than average.
Two-thirds of the people in this group are over 55.