Consensus is the coming to an agreement on something by multiple parties.
Within the context of cryptocurrency, digital assets, and blockchain, consensus is the mechanism by which a network operates and guarantees the validity of the activity that happens on it. Consensus mechanisms are fault-tolerant processes that establish the single true valid state of a ledger.
Public blockchains operate as decentralized, self-regulating systems on a global scale without any single authority. Because there is no final single party to pass judgment on the validity of data, the chains involve contributions from hundreds of thousands of participants who work on verification and authentication of transactions (miners/validators/nodes) occurring on the blockchain.