Double spending is a potential error in a digital cash system where the same individual digital token can be spent more than once. Solving this double spending problem through the Proof of Work mechanism laid the foundation for cryptocurrencies like Bitcoin.
In traditional financial systems, physical cash prevents the double spending problem because when you give someone a banknote, you cannot simultaneously give the same note to someone else. However, in the digital world where transactions occur electronically, there is a risk that the same digital currency unit can be copied and spent multiple times. Typically, to prevent double spending, a central trusted entity is used. However, this introduces both availability and trust issues, creating a "single point of failure."
Satoshi Nakamoto's resolution of the double spending problem
With a unique combination of cryptographic functions (hashing) and a distributed ledger, Nakamoto described in his Bitcoin whitepaper the decentralized solution to the double spending problem for the first time. The chaining of transaction blocks ("blockchain") creates a unique history that can only be altered with a majority of the network's computing power.
Attempts at double spending are prevented as the decentralized network of nodes (computers) continuously verifies the integrity of transactions. Any attempt to duplicate the same digital currency unit for multiple transactions is rejected by the network. Preventing double spending is a fundamental feature that underpins the credibility and reliability of digital currencies, making them suitable for use as money in the digital age.