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English definition of “five nines”

five nines

noun [plural]     (also five nines reliability) IT
99.999 percent: the percentage of time that a computer system should work without failing: In the computer industry, the challenging target is "five nines" or 99.999% - an average annual downtime of five minutes. five nines availability/uptime
(Definition of five nines noun from the Cambridge Business English Dictionary © Cambridge University Press)
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