51 percent attack

A 51 percent attack occurs when a single entity controls more than half of the network’s total hash rate. This allows the attacker to create a competing blockchain that outpaces the honest chain, enabling double spending and transaction censorship. The attack does not permit creation of invalid coins or breaking of cryptography but undermines confidence in the immutability of the ledger. Today, a 51 percent attack is practically inconceivable given the immense computational power it would require to outpace Bitcoin's current hashrate.