What is an overstamp on currency?

A "restrike" on coins is when the coin is struck again using the same dies. An "overstrike" on coins is when the coin was struck again with a different die to change the appearance of the coin. A "counterstrike" on coins is when the coin is restruck using a different die. For example, the British "counterstruck" 18th Century Spanish American 8 Reale coins and called them Dollars. This was once a relatively common practice. An "overdate" is a number engraved in the date over the original engraved date.