A 51% attack is an attack on a blockchain by a group of miners who control more than 50% of the network's mining hash rate. Attackers with majority control of the network can interrupt the recording of new blocks by preventing other miners from completing blocks.
A 51% attack will allow the attackers to create a flawed block containing transactions that never happened. Such an attack can essentially be used to print new coins on to the particular Blockchain and add them to the attacks account/wallet.
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