You forgot to mention that nodes always consider the longest block chain to be the consensus. A 51% (majority) attack is when one miner is able to produce more

You forgot to mention that nodes always consider the longest block chain to be the consensus. A 51% (majority) attack is when one miner is able to produce more blocks than the rest of the network, therefore controlling consensus. Such a miner could do a double-spend transaction by first spending on the short chain and then reverting his transaction on the longest chain. See https://en.bitcoin.it/wiki/Majority_attack https://en.bitcoin.it/wiki/Majority_attack