noun
flat tax
A tax system where everyone pays the same tax rate regardless of income level.
フラットタックスは、所得に関係なく一律の税率を適用する制度です。
A flat tax is a system that applies a uniform tax rate regardless of income.
Variant with interpunct; less common but seen in some texts.
累進課税 is a progressive tax system where higher income brackets are taxed at higher rates, unlike フラットタックス which uses a single rate.
Borrowed from English 'flat tax'.