Definition of a 51% Attack
A 51% attack refers to a potential security threat to the Bitcoin network. In this type of attack, a single validator (miner) or a group of miners (mining pools) controls more than 50% of the computational power (hash rate) of the entire Bitcoin network. This would enable the attackers to alter the network’s rules, such as rewards, block sizes, or individual transactions.
Is it a threat to Bitcoin?
Such an attack would require an enormous amount of computational power and resources, which may not be proportional to the potential gains.
Since the value of the network is rooted in user trust, stability, and utility, miners are also interested in maintaining an intact network. Even if 51% of the global computational power could be used by an entity to make changes to transactions in their favor, users with their network copies (nodes) would choose to use a new Bitcoin network or an older version of the network that ignores the recent blocks. As a result, the bitcoins from the old network branch would become nearly worthless because no one would be willing to exchange them for goods and services or other means of exchange.
Therefore, based on current knowledge, a 51% attack is not expected to pose a threat to the Bitcoin network.