Reply #6: It's not defined.

[TahitiNut]

Generally, I take it to mean the middle three quintiles of income.

If we go by marginal tax rates, I dunno. In 2003

Rate Percent
0 19.8 percent of returns paid a top marginal rate of 0%
10 20.3 percent of returns paid a top marginal rate of 10%
15 37.0 percent of returns paid a top marginal rate of 15%
25 17.9 percent of returns paid a top marginal rate of 25%
28 3.0 percent of returns paid a top marginal rate of 28%
33 1.2 percent of returns paid a top marginal rate of 33%
35 0.8 percent of returns paid a top marginal rate of 35%

From that, I'd call 'middle class' income between $28,400 and $143,500.