51% Attack
Taking over more than 50% of a PoW network's hash power.
If an attacker holds >51% PoW hash rate, they can reverse their own transactions (double spend), censor others, but CANNOT alter old blocks or print coins. Bitcoin is practically immune - attack would cost ~30B USD/day. Victims: Ethereum Classic 2019 (5 attacks, 1.1M USD lost), Bitcoin Gold, Verge.
In plain words
If someone seized more than half the network's miners, they could temporarily rewrite rules - spend the same crypto twice. Bitcoin is too big to attack.
Example
Ethereum Classic 2020: three 51% attacks in 1 month, 5.6M USD double-spent.
Also known as: majority attack