Billon is a mixture of silver and copper and sometimes other metals. The silver content is always less than the copper. The word billon comes from the middle Latin billio and means "a largely copper containing coin" or also simply "unit of payment".

In the 14th to the 17th centuries the billon coin called the plack circulated in the Netherlands.

Later in the middle ages and in more modern history billon coins are considered as an example of devalued and deflated coins. However, this coinage was used by the common people in daily transactions and its importance in everyday life is now becoming recognized

Billon had also been used in ancient times. Several cities on the island of Lesbos manufactured billon coins that contain 40% silver and 60% copper during the 6th and 5th century BC.