Majority Attack

51% attack

1 [attack]
Controlling over 51% of the network and using it to attack the network. By having over half of the total hash power, it's possible to manipulate a blockchain in ways such as reversing transactions, and blocking transactions to/from specific addresses.
2 [risk]
This type of attack is unlikely to happen to any major chain with significant hash power, but smaller chains with low hash power are vulnerable to such an attack
Since the risk of a 51% attack decreases as support for a currency increases, the higher the hash power and the number of stakeholders for a chain, the more secure it becomes.