An attack on a network of certain cryptocurrency which allows the attacker to create fraudulent transactions, also known as “double spend” transactions. This is possible by means of controlling more than 50% of the network’s hash rate/mining power.
The 51% attack concept was first explained for Bitcoin, but could be undertaken on many blockchains. To date only Krypton and Shift, two blockchains based on Ethereum, suffered 51% attacks in August 2016.
A 51% attack on Bitcoin remains hypothetical and unlikely, but considering the consolidated nature of the mining pool for Bitcoin it is a possibility as a group of miners could collude to control more than 50% of the network’s computing power. In a case such as this the attackers would be able to block new transactions from being confirmed on the network, in effect preventing some or all users from receiving funds. It would also be possible for them to reverse transactions as long as they are in control of the network, meaning they could double-spend coins.
It’s not likely that a 51% attack would destroy Bitcoin if it occurred, because theoretically the attackers would not be able to create new coins or make changes to old blocks. That said, it would almost certainly be extremely damaging, perhaps enough so to forever crush the trust that has been built up by Bitcoin.