Currency board
Currency board is an exchange rate regime in which a country's exchange rate maintain a fixed exchange rate with a foreign currency, based on an explicit legislative commitment. It is a type of fixed regime that has special legal and procedural rules designed to make the peg "harder—that is, more durable". Examples include the Hong Kong dollar against the U.S dollar and Bulgarian lev against the Euro.
https://en.wikipedia.org/wiki/Exchange_rate_regime