Byzantine fault tolerance
In a large distributed systems, it is very likely that some components will fail. Robustness of distributed systems therefore requires that the behaviour of the systems is unaffected by the failure of a fraction of its components.
The hardest type of failure is when the component behaves arbitrarily badly and not simply crashes and stops responding to the other components of the system. This type of failure is referred to as "Byzantine" as introduced by Lamport, Shostak, and Pease in 1982 []