What is 51 attack in Blockchain

Question

Though Blockchain is said to be secure, I heard that it can be vulnerable to 51% attack? What does this mean?

Answer 1

51% Attack is a situation where a group of miners who hold more than 50% of the Network Hash Rate could manipulate with the New transactions or able to reverse the transactions that were recently confirmed and kind of doing Double spend. It is Highly unlikely to be able to do that today but it is possible. Usually for the Block to get verified, the majority of the network should accept it. So if the majority(at least 51%) is corrupt, then the Blockchain is not secure anymore.