What Is ‘Double-Spending’ and Why Is It a Core Concern of a 51% Attack?

Double-spending is the act of successfully spending the same unit of digital currency more than once. In a 51% attack, the attacker first sends a transaction (e.g. a deposit to an exchange).

Then, using their majority hash power, they mine a secret, longer chain that excludes the original transaction, and broadcast it, replacing the first chain. The exchange sees the original deposit reversed, but the attacker retains the funds, effectively spending them twice.