Byzantine Fault Tolerance, or BFT, refers to the ability of a system to still function correctly if some of its components fail, or behave maliciously, even when there is imperfect information regarding which components are working correctly.
In the context of blockchains, it is important for the consensus mechanism to have this property. For EOSIO specifically, the DPoS consensus mechanism should work even if some of the Block Producers fail or misbehave.

The term Byzantine comes from a computer science problem , the Byzantine Generals’ Problem, in which a group of army commanders encircling a city have to agree on a plan, while some of the generals may be disloyal.