What Is “Double-Spending” in the Context of a 51% Attack?

Double-spending is the act of successfully spending the same cryptocurrency twice. In a 51% attack, the attacker first makes a legitimate transaction to a merchant or exchange.

Then, using their majority hash power, they mine an alternative, secret version of the blockchain that does not include the original transaction. Once the secret chain is longer, they broadcast it, forcing the network to adopt it, effectively reversing the original transaction and allowing the attacker to spend the coins again.