The ECU was the European monetary system's composite unit of account before the third stage of Economic and Monetary Union. Its value derived from specified quantities of the currencies of 12 EU Member States, although the Union had 15 members at the time. Changes in the exchange rates of those currencies altered the ECU's value according to their respective weights. The euro replaced the ECU on 1 January 1999, with one ECU converted into one euro.
Source: European Central Bank (ECB) (source). This glossary entry is an independently worded adaptation of the cited information.