From what I understood, it is the ability of a miner with 51% hashing power of the whole network to be able to control bitcoin transactions.
the thing about 51% attack that i have learned is the fact that 51% is the border line of the limit that you should have to start doing the attack not the amount that you can pull it off with. for that you probably need more than 70% or possibly even more.
That's the thing 51% doesn't mean that they will replace the original chain but they are somehow eligible to do it if their attack would be more stronger.
Now, fast forward to today, this attack is again looking possible because, from what I read, the segwit2x was being pushed by a group of miners, that, if combined, will result to 80% of the hashing power of the world.
It will be a threat but still there isn't any confirmation about the combining of miner's group which will make around 80% of the total hash power because there is a contradiction between them too.