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

Double-spending is the act of successfully spending the same cryptocurrency units more than once. It is the main concern of a 51% attack because an attacker controlling the majority of the hashrate can create a private, longer chain where they reverse a transaction (e.g. a payment to an exchange) after it has been publicly confirmed.

This allows them to spend the coins twice, undermining the fundamental security promise of the blockchain.