The ability for a computer, or group of computers, to approve more than 50% of the transactions on a blockchain network. This is typically of concern on a Proof-of-Work system like Bitcoin or Litecoin.
If this happens, the ability exists to reverse transactions, spend coins more than once (double-spend), and manipulate the blocks that are added.
A possible solution to this is to fork the blockchain and reverting back to the original blocks at the time of the attack.
In cryptocurrency forking and resetting coin distribution is a controversial matter. This happens when the DAO on Ethereum was penetrated and resulted in the creation of Ethereum Classic. In this situation, a flaw in the smart contract allowed for the hack.
One of the keys to blockchain is the fact it is distributed ledger technology. This means the ledger of transactions puts it on par with banks and other financial institutions. These entities all maintain their own ledger.
A decentralized network still is responsible for the integrity of the ledger. This is aided by the immutable aspect of blockchains. Once a block is added to the chain, the transactions are not reversible. A 51% attack makes this possible. Here is where it is vital for blockchains to avoid having the ledger altered.
In September 2022, Ethereum switched from PoW to a Proof-of-Stake (PoS) mechanism. This was done in an effort to scale the network and handle more transactions. Many also complained about the high transaction fees associated with PoW. This will be addressed in upcoming hard forks.
Thus, a 51% attack on Ethereum, at least in the traditional sense, is no longer possible.
51% Attack on PoS?
Can there be a 51% attack on a Proof-of-Stake (PoS) blockchain. Not in the sense of how it is defined here. However, that can be attacks to take over a blockchain.
Justin Sun launched one against Steem in 2019. He was able to acquire a pre-mine stake that was available off the open market. This game him significant control over the block producers. Ultimately, he got a hold of the chain but the community forked.
Under this consensus mechanism, an attack is possible although it is not terms a 51% attack. To fully take control, more than 50% of the coin distribution has to be in the hands of an individual or group seeking to attack. This would be more of a money attack as opposed to computing.