A distributed system is one in which the failure of a computer you didn't even know existed can render your own computer unusable.
Email by Leslie Lamport in May 28th 1987.
Leslie Lamport got his 2013 Turing Award for his work on distributed systems.
This quote refers to the need of a distributed system to count on resilience and redundancy in its design; and not just focus on the dispersion and communication of its components.
I recalled this quote during December 2021 Amazon Web Services (AWS) outage where plenty of business saw their operations affected. It was particularly interesting that the iRobot's line of vacumm cleaner robots was affected.