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Wiktionary
hidden tax

n. (context taxation English) a tax that is not visible to the end consumer (retail consumer)

WordNet
hidden tax

n. a tax paid unwittingly by the consumer (such as ad valorem taxes)

Wikipedia
Hidden tax

A hidden tax is a tax that is not visible to the taxpayer. These taxes can raise prices of goods and lower salaries for workers. Hidden taxes, although hidden, can decrease the purchasing power of individuals significantly.

Many kinds of tax behave like hidden taxes:

Corporate income tax: Because of this tax, shareholders and employees get less dividends and salary.
Tariffs or import taxes: It lowers competition by raising the price of imports and products produced outside.
Sin taxes: Taxes on alcohol and cigarettes are highly regressive.
Travel taxes:
Utilities taxes: According to a 1998 study, taxes take up 48% of the price of gasoline.
Inflation tax: It is a regressive hidden tax.
Financial repression: A range of measures which governments can employ to reduce their debt, which are often accompanied by inflation.
Profits from government-owned corporations: Especially when a government-owned corporation enjoys a legally protected monopoly and records excessive profits, such profits may be criticized as hidden taxation. Governments in some jurisdictions directly own utilities (making such profits related to utility taxes) and some monopolize the alcohol trade (making such profits related to sin taxes).
Sales taxes such as turnover taxes

Usage examples of "hidden tax".

So in the end, every airline passenger pays a few dollars extra for their ticket, in a hidden tax.