What Is Double-Spending and Why Is a 51% Attack Necessary to Execute It?

Double-spending is the act of successfully spending the same cryptocurrency twice. It is prevented by the blockchain's chronological and consensus-driven nature.

A 51% attack is necessary because it grants the attacker the power to control the transaction ordering and confirmation. The attacker can send a transaction, receive goods, and then use their majority hash power to create a longer, alternative chain that omits the initial transaction, effectively reversing it.