A 51% Attack is an attack on a blockchain network where an entity or group of miners controls more than 50% of the network's hash power. This allows the attacker to control the majority of the processing of transactions or new blocks inserted into the blockchain. With this control, the attacker can perform several malicious actions, including :

• Double Spending: Attackers can send their own coins or tokens, spend them, and then undo that transaction by hacking an existing block, or even creating a longer alternative chain.
• Preventing Confirmations: Attackers can prevent certain transactions from being confirmed or slow down the confirmation of transactions, creating uncertainty in the reliability of the network.
• Censorship: An attacker can choose not to include transactions from certain parties into a block, affecting overall access to the network.

These attacks capitalize on weaknesses in the Proof of Work (PoW) consensus mechanism used by many blockchains such as Bitcoin and Ethereum. In response, blockchain protocols have developed additional protection mechanisms, such as more transaction confirmations or a transition to stronger consensus protocols like Proof of Stake (PoS), to reduce the risk of such attacks.