all hail wikipedia...
http://en.wikipedia.org/wiki/American_middle_class
The American middle class is an ambiguously defined social class in the United States.[1][2] While concept remains largely ambiguous in popular opinion and common language use,[3][4] contemporary sociologists have put forth several, more or less congruent, theories on the American middle class. Depending on class model used, the middle class may constitute anywhere from 45% to 49% of households. Sociologists such as Dennis Gilbert of Hamilton College commonly divide the middle class into two sub-groups. Constituting roughly 15% to 20% of households is the upper or professional middle class consisting of highly educated, salaried professionals and managers. Constituting roughly one third of households is the lower middle class consisting mostly of semi-professionals, skilled craftsmen and lower level management.[5][2] Middle class persons commonly have a comfortable standard of living, significant economic security, considerable work autonomy and rely on their expertise to sustain themselves.[6]