51% Attack refers to a theoretical attack that could happen if a person or an organisation manages to collect 51% of the computing (hashing) power on the network. The attackers would be able to cancel and reverse transactions, which would allow double-spending. There is a website that shows the cost of a one hour 51% attack for different cryptocurrencies. According to the website the current cost of conducting a 51% attack on Bitcoin would be $234,741 per hour. Also the attacker should be able to sustain 51% of the Bitcoin hash rate which is massive amount of mining power that is not available to buy even if you had the money. This is why Bitcoin is referred to most resistant and unhackable coin. 51% attack can only be done with smaller altcoins.