Ask HN: What is a machine year?

I just read a paper: 'Paxos Made Live - An Engineering Perspective' by Tushar et al. It mentions "machine years" in it. What is a machine year? Sentence: "A small number of failures in 100 machine years would be considered excellent behavior for most production systems." I googled it, but no answer. Link to the paper: http://static.googleusercontent.com/media/research.google.com/en//archive/paxos_made_live.pdf