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Definition: five nines

Refers to 99.999% uptime that many network providers are fond of stating as the reliability of their networks. Whether five nines is truly what is delivered at all times may be debatable, because that means no more than 26 seconds of downtime per month. In any event, five nines means that everything in the datacenter is fully redundant, including AC power, air conditioning, network equipment and computer hardware.

From One Nine to Nine Nines
Nines ratings range from a maximum of 72 hours of downtime per month (one nine) to no more than 2.6 milliseconds per month (nine nines). See failover and redundancy.