It means "arbitrary". Byzantine faults refer to "any arbitrary behaviour by the machine". The term was coined by Leslie Lamport in the early 80s when he defined the Byzantine Generals Problem

The Byzantine generals problem:

https://en.m.wikipedia.org/wiki/Byzantine_fault_tolerance

It means that not only the machines can fail by shutting down - they can send incorrect data.

I recommend this: http://techtv.mit.edu/genres/19-engineering/videos/16444-pra... which helped me learn how it works ~6 years ago :)