The term 51% attack refers to a scenario in Proof of Work (PoW) blockchain consensus mechanisms where over half of the hash power (essentially 51%) is controlled by a malicious actor who is thus able to disrupt the blockchain’s functionality.
The attacker could manipulate transactions, for example, by preventing the confirmation of transactions, changing their order, or double spend certain transactions. Even though theoretically all blockchain networks using proof-of-work technology can be affected by 51% attacks. Especially smaller networks, where the hashing power is concentrated in the hands of a few miners, can be targeted more easily by attackers.
For example, BTG, VTC, XVG and EXP were affected by a 51% attack in the past.