We don't have synonyms for flat tax yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are fat tax, flat-tax and flat cap.
(n) :
(government, taxation) A tax, usually income tax, whose percentage rate or currency value remains constant, regardless of the amount to which the tax is applied.
(n) :
(government, taxation) A tax, usually income tax, whose percentage rate or currency value remains constant, regardless of the amount to which the tax is applied.