What Is a “51% Attack” and How Does PoW Prevent It?

A 51% attack is a hypothetical scenario where a single entity or group gains control of more than 50% of a blockchain network's total hashing power. This control would allow them to prevent new transactions from gaining confirmations and, more critically, to reverse their own transactions, enabling double-spending.

PoW prevents this by making the acquisition and maintenance of such a large amount of computing power prohibitively expensive and economically unviable.