51% Attack
Also called: 51 percent attack, majority attack, double spend attack
A 51% attack is one party controlling most of a network's mining power or stake, allowing them to reverse recent transactions and double-spend. It cannot rewrite deep history or steal from arbitrary addresses. Large chains are effectively immune on cost grounds, while smaller proof-of-work chains have been attacked repeatedly — the security budget scales with the value being secured, and so does the attack cost.