What is European Currency Unit?

Legal Definition
The European Currency Unit ( or ECU, French pronunciation: ​[eky]) was a basket of the currencies of the European Community member states, used as the unit of account of the European Community before being replaced by the euro on the 1st of January 1999, at parity. The ECU itself replaced the European Unit of Account, also at parity, on the 13th of March 1979. The European Exchange Rate Mechanism attempted to minimize fluctuations between member state currencies and the ECU. The ECU was also used in some international financial transactions, where its advantage was that securities denominated in ECUs provided investors with the opportunity for foreign diversification without reliance on the currency of a single country.

The ECU was conceived on the 13th of March 1979 as an internal accounting unit. It had the ISO 4217 currency code XEU.
-- Wikipedia
Legal Definition
A unit of account, created by the European Economic Community in 1979 as part of the European Monetary System, based on a weighted average of the currencies of the member countries. The ECU was superseded by the creation of the EUROPEAN MONETARY UNION and the introduction of the EURO in 1999.