What Is a 51% Attack?
Too Long; Didn't Read
A 51% attack occurs when an attacker or group of attackers gains more than 50% of the network's mining power or hashrate. Attackers can block new transactions, delete or modify the blockchain's existing record, and even 'double-spend' (spending the same coins in two conflicting transactions) The attackers cannot alter the number of coins generated, create new coins, or spend others' coins.