Double-spending - Definition from the Hotspot Shield Encyclopedia
Double Spending is a recurring problem that many digital currencies have to address and solve problems too. With Fiat currencies, you have physical currency that cannot be duplicated. Once it is spent, the owner no longer has possession and it is impossible to spend that same piece of currency again.
With digital currencies, there is no physical ownership of the currency or an actual item that can be traded. This means that digital currencies can be duplicated and in certain cases spent twice. There are many implementations and technologies developed that prevent digital currencies from being spent twice and a cryptocurrency will have safeguards and checking methods such as proof-of-stake to ensure that this cannot occur.