Byzantine Fault Tolerance
A Byzantine fault tolerant (BFT) system keeps working correctly even if some participants fail or act maliciously.
In depth
The name comes from a thought experiment about generals who must coordinate an attack while some of them may be traitors sending conflicting messages. Blockchain consensus protocols are BFT: they guarantee agreement as long as fewer than a third (for classic BFT) or half (for PoW) of the power is dishonest. It is the formal backbone of every consensus mechanism.
Why it matters
BFT is what turns "many computers" into "one trustworthy ledger" despite bad actors.