investopedia.com/terms/f/flattax.asp
Definition of 'Flat Tax'
A system that applies the same tax rate to every taxpayer regardless of income bracket. A flat tax applies the same tax rate to all taxpayers, with no deductions or exemptions allowed.
imf.org/external/pubs/ft/wp/2006/wp06218.pdf
Particularly in the United States, the term "flat tax" is associated with Hall and Rabushka (1983 and 1985; HR). Their proposal is for a very precisely defined and coherent tax structure: a combination of a cash-flow tax on business income and a tax on workers' income, both levied at the same, single rate (with a personal allowance available against the wage tax).
cato.org/sites/cato.org/files/serials/files/cato-journal/1985/11/cj5n2-14.pdf
A true fiat rate tax on income has two characteristics: first, the tax base is a comprehensive measure of income with no preferential
treatment given to specific sources or uses of income, and second, a single tax rate is applied to that base. In short, a flat rate tax is a proportional tax on total income.
en.wikipedia.org/wiki/Flat_tax
A flat tax (short for flat tax rate) is a tax system with a constant marginal rate, usually applied to individual or corporate income. A flat tax falls under proportional tax as they allow certain deductions. There are various tax systems that are labeled "flat tax" even though they are significantly different.
Barney, unless you are talking about something biblical, every credible source I came across refers to Flat Tax as a rate to be levied, not a set amount per person regardless of income / sources of income.
Exodus 30:11-16:
11 And the LORD spake unto Moses, saying,
12 When thou takest the sum of the children of Israel after their number, then shall they give every man a ransom for his soul unto the LORD, when thou numberest them; that there be no plague among them, when thou numberest them.
13 This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary: (a shekel is twenty gerahs:) an half shekel shall be the offering of the LORD.
14 Every one that passeth among them that are numbered, from twenty years old and above, shall give an offering unto the LORD.
15 The rich shall not give more, and the poor shall not give less than half a shekel, when they give an offering unto the LORD, to make an atonement for your souls.
16 And thou shalt take the atonement money of the children of Israel, and shalt appoint it for the service of the tabernacle of the congregation; that it may be a memorial unto the children of Israel before the LORD, to make an atonement for your souls.