A flat tax is a single-rate tax on income (and possibly on other things as well). The idea is to have as wide a tax base as possible, so for a given amount of tax revenue collected, the marginal rate will be lower than in a non-flat tax; this is intended to boost economic activity.

Few weeks in modern US history have been as momentous as this last one. It saw two events that are likely to have a huge impact on the way Americans see themselves and how they are governed.
First, there was Katrina. The whole country, the whole world, has watched in disbelief as a great American city [...]