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 funds more than once. This is possible in a 51% attack because the attacker controls the majority of the network's hashrate, allowing them to confirm a transaction to a merchant and then later create a longer, secret chain that invalidates the first transaction and sends the funds back to themselves.

It undermines the fundamental trust and immutability of the blockchain.