51% attack or majority attack refers to a malicious actor or a group that attacks the blockchain system. They do it by successfully controlling more than 50% of the total hashing power. The impact of a 51% attack potentially causes network disruption.
As we know, blockchain networks work through a consensus mechanism. When the attacker controls the majority of the hashing power and takes over the consensus, they can intentionally modify a transaction, preventing some or all transactions and even double-spending.
The risk of a 51% attack is higher to occur on blockchain networks with low hashing power. It is because the attackers can easily meet the majority of computing power required. Thus, the more miners participated and the more resources spent, the safer the network will be from 51% attacks. Bitcoin has become the safest blockchain as it has enormous hashing power.
Find more about 51% attack and how it works in this article.