Tobin tax (Wikipedia, Economist's Glossary)

A Tobin tax is a small tax (0.1% to 0.25%) on all trade of currency across borders. It really requires an international institution (not a government acting alone) to implement. Read the following paper if you want to learn about a similar tax unilaterally imposed by a single government:

Efficient Capital Controls [link]

Rodney Schmidt, International Development Research Centre, Government of Canada
December 2000

Abstract: "Controls on short-term capital inflows or panic-driven capital outflows may benefit emerging markets with fragile financial sectors and adjustable-peg currency regimes. However, the controls seen so far are relatively easy to evade, often complex and obscure, and supported by large corruptible bureaucracies."

A tax on foreign-exchange payments avoids these drawbacks. It is
transparent, inexpensive to set up and operate, administratively lean,
and easy to adjust. A Tobin tax in effect, it is enforceable even when
applied unilaterally."