Blockchain Consensus
The process by which all nodes in a blockchain network agree on the valid state of the ledger.
What is Consensus Mechanism?
A blockchain consensus mechanism is the set of rules and algorithms that allow all participating nodes in a decentralized network to agree on which transactions are valid and in what order they occurred — without relying on a central authority. Consensus mechanisms solve the double-spend problem inherent in digital money: they ensure that no coin can be spent more than once even when no single entity controls the ledger. The two dominant approaches are proof-of-work, used by Bitcoin, which requires expensive computational effort to add blocks, and proof-of-stake, adopted by Ethereum and many newer networks, which selects validators based on the amount of cryptocurrency they lock up as collateral. Other mechanisms include delegated proof-of-stake, proof-of-history, and practical Byzantine fault tolerance, each offering different trade-offs between security, speed, and decentralization.
Example
Bitcoin's proof-of-work consensus requires miners to expend real-world energy to solve cryptographic puzzles. This computational cost makes it prohibitively expensive to rewrite history — an attacker would need to control over 50% of the global hash rate. This 'Nakamoto consensus' underpins Bitcoin's security model.