Consensus is the rule system that gets participants on a network to agree that "this transaction/block is valid." With no central admin on the network, consensus is what decides which record counts as the truth.
Without consensus, one person might have ledger version A while another has version B, and the network could split apart. Consensus is the mechanism that builds a shared truth.
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Consensus = the rule set a decentralized network uses to agree on what's true.