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A Tobin tax, suggested by Nobel Memorial Prize in Economic Sciences Laureate economist James Tobin, was originally defined as a tax on all spot conversions of one currency into another. Tobin's original tax was intended to put a penalty on short-term financial round-trip excursions into another currency. By the late 1990s, however, the term Tobin tax was being incorrectly used to describe all forms of short term transaction taxation, whether across currencies or not - another term for these broader tax schemes is Robin Hood tax due to tax revenues from the (presumably richer) speculator funding general revenue (of whom the primary beneficiaries are poorer). More exact terms however apply to different scopes of tax. One non-tax regulatory equivalent of Tobin's (very narrow original) tax is to require "non-interest bearing deposit requirements on all open foreign exchange positions.". If these deposit requirements result in forfeits or losses if a currency suddenly declines due to speculation, they act as inhibitions against deliberate speculative shorts of a currency. However, they would not raise funds for other purposes, so are not a tax.
Source: Wikipedia - Tobin tax