A 51% attack refers to an attack on a Proof-of-Work (PoW) blockchain where an attacker or a group of attackers gain control of 51% or more of the computing power or hash rate. PoW is a system of consensus used by blockchains to validate transactions.
Why is there a limited amount of Bitcoin?
Bitcoin also has a stipulation—set forth in its source code—that it must have a limited and finite supply. For this reason, there will only ever be 21 million bitcoins ever produced. On average, these bitcoins are introduced to the Bitcoin supply at a fixed rate of one block every ten minutes.