What is 51% Attack
51% attack is a scenario in which the security of a blockchain network is compromised because a single entity or multiple entities working together possess 51% or more of the network’s mining potential. In that case, double spending is possible.
The attacker or attackers cannot stop or undo the transactions of others, but they can halt the transactions from being verified, disabling the network. Moreover, it is unlikely for the malicious entity to be capable of taking cryptocurrency which hadn’t previously been in his possession.
Past transactions which have been followed by several transactions in the chain are more difficult to reverse.
As the network expands and the number of nodes increases, the scenario becomes less and less likely. Larger networks such as Bitcoin are considered quite safe, but some smaller networks, such as Bitcoin Gold, in the past have fell victims of 51% attacks.