Byzantine Fault Tolerance (BFT)
In short: Byzantine Fault Tolerance (BFT) is the property that allows a distributed computer system to reach consensus on its own, without a central coordinator…
Byzantine Fault Tolerance (BFT) is the property that allows a distributed computer system to reach consensus on its own, without a central coordinator, even if some of its components fail or deliberately send false information. The name comes from the Byzantine Generals Problem, a thought experiment about commanders who must agree on a plan although some of them may be traitors.
Why it matters
A blockchain is run by many independent nodes that do not trust each other, so it needs BFT to keep one agreed history. Classic BFT protocols keep working as long as faulty or dishonest participants make up less than one third of the network. Proof of Work and Proof of Stake reach Byzantine fault tolerance in different ways, and many PoS chains use BFT-style voting to finalize blocks. Without this property, a few misbehaving nodes could split the network or push through invalid transactions.
Example
Picture four validators voting on the next block, one of which is compromised and tells some peers "yes" and others "no". A BFT protocol accepts a block only when enough matching votes are collected, here three of the four. The three honest validators therefore still agree on the same block, and the lying node cannot create two competing versions of history.