A 51% Attack happens when the majority of the cryptocurrency network’s hash rate or validation authority is controlled by a single person or entity.
Also known as a majority attack, malicious actors could use a 51% attack to cause disruption on the network, potentially overriding the consensus mechanism of the network.
Smaller networks are more susceptible to 51% attacks, as they have fewer participants, making it easier for a single person to gain control over more computing power compared to all other network participants.
51% attacks, while possible, are considered extremely unlikely, especially for larger networks like Bitcoin and Ethereum , because of the high cost of obtaining that much hash power.
Bitcoin SV, Bitcoin Gold and Ethereum Classic (ETC) all suffered 51% attacks.