i've reworded the lead to try and reduce the ambiguity in the definition hopefully this helps the common definition of a flat tax which is mostly used to imply income taxation as oppose to consumption or a sales tax is usuall not a horizontal line in regard to average tax rate and progressivity certainly the term of flat tax implies one rate but in practice and legislation it may be one rate but exempts income up to a certain level so they don't count this is of course regarding a marginal tax rate view and when such an exemption is applied to an average tax rate it becomes a progressive tax talk june utc